Add timemanager.ui from glade and start gtk app by it.

Makefile

+CC = gcc
+CFLAGS = `pkg-config --cflags gtk+-3.0`
+LIBS = `pkg-config --libs gtk+-3.0`
+
+all: timemanager
+
+timemanager: main.c
+	$(CC) $(CFLAGS) $< -o $@ $(LIBS)
+
+.PHONY: clean
+clean:
+	rm *.o timemanager

main.c

+#include <stdio.h>
+#include <time.h>
+#include <string.h>
+
 #include <gtk/gtk.h>
 
+#define GTK_EVENT_ACTIVATE "activate"
+#define GTK_EVENT_DESTROY "destroy"
+#define GTK_EVENT_CLICKED "clicked"
+
+#define DEBUG
+
+/* Time info buffer */
+static time_t time_now;
+static struct tm *tm_now;
+
+/* Timezone info buffer */
+#define MAX_TIME_STATUS_LINE 8
+static char timezone_cur[32];
+static char timezone_buffer[512][32];
+static int timezone_num = 0;
+
+/*
 static void print_hello(GtkWidget *widget, gpointer data) {
     g_print("Hello World\n");
 }
@@ -23,15 +44,263 @@ static void activate(GtkApplication *app, gpointer user_data) {
 
     gtk_widget_show_all(window);
 }
+*/
+
+static int get_timezone(const char *src, char *timezone) {
+    const char *buf = src;
+    int index = 0;
+
+    /* Skip space char */
+    while(buf[index] == ' ') index++;
+
+    for (int i = 0; ; i++, index++) {
+        if (buf[index] != ' ') {
+            timezone[i] = buf[index];
+        } else {
+            /* End by space */
+            timezone[i] = 0;
+            break;
+        }
+    }
+
+    return 0;
+}
+
+static void get_time_date_status() {
+    char time_status[8][32];
+    FILE *fp = popen("timedatectl status", "r");
+
+    for (int i = 0; i < MAX_TIME_STATUS_LINE; i++) {
+        char *buf = fgets(time_status[i], 32, fp);
+        if (buf == NULL) {
+            break;
+        } else if (strstr(time_status[i], "Time zone") != NULL) {
+            int line_size = strlen(buf);
+            fseek(fp, line_size, SEEK_CUR);
+
+            buf = strchr(time_status[i], ':');
+            if (buf != NULL) {
+                get_timezone(buf + 1, timezone_cur);
+            }
+        }
+    }
+#ifdef DEBUG
+    g_print("[DEBUG] Current timezone: %s\n", timezone_cur);
+#endif
+    pclose(fp);
+}
+
+static void timezone_init(GtkBuilder *builder) {
+    GObject *combo_timezone;
+    GtkListStore *store = NULL;
+    GtkTreeIter iter;
+    GtkCellRenderer *cell = NULL;
+
+    combo_timezone = gtk_builder_get_object(builder, "combobox-timezone");
+    if (combo_timezone == NULL) {
+        g_print("[ERROR] can not get combobox timezone object\n");
+        return;
+    }
+
+    FILE *fp = popen("timedatectl list-timezones", "r");
+
+    timezone_num = 0;
+    for (int i = 0; i < 512; i++) {
+        char *buf = fgets(timezone_buffer[i], 32, fp);
+        if (buf == NULL) {
+            break;
+        } else {
+            int size = strlen(timezone_buffer[i]);
+            timezone_buffer[i][size - 1] = 0; // remove '\n' char.
+            fseek(fp, strlen(buf), SEEK_CUR);
+            timezone_num++;
+        }
+    }
+
+    pclose(fp);
+
+#ifdef DEBUG
+    g_print("[DEBUG] Get %d timezones\n", timezone_num);
+#endif
+
+    store = gtk_list_store_new(1, G_TYPE_STRING);
+
+    for (int i = 0; i < timezone_num; i++) {
+        gtk_list_store_append(store, &iter);
+        gtk_list_store_set(store, &iter, 0, timezone_buffer[i], -1);
+    }
+
+    gtk_combo_box_set_model(GTK_COMBO_BOX(combo_timezone), GTK_TREE_MODEL(store));
+    
+    cell = gtk_cell_renderer_text_new();
+    gtk_cell_layout_pack_start(GTK_CELL_LAYOUT(combo_timezone), cell, TRUE);
+    gtk_cell_layout_set_attributes(GTK_CELL_LAYOUT(combo_timezone), cell, "text", 0, NULL);
+}
+
+static void date_activate(GObject *entry, gpointer user_data) {
+}
+
+static void button_handler(GObject *button, gpointer user_data) {
+    gtk_main_quit();
+}
+
+static void date_init(GtkEntry *entry_date, GtkEntry *entry_hour, GtkEntry *entry_minute, GtkEntry *entry_second) {
+    gchar date_text[11];
+    gchar hour_text[3];
+    gchar minute_text[3];
+    gchar second_text[3];
+
+    time(&time_now);
+    tm_now = localtime(&time_now);
+
+    sprintf(date_text, "%04d-%02d-%02d", (1900 + tm_now->tm_year), tm_now->tm_mon, tm_now->tm_mday);
+#ifdef DEBUG
+    g_print("[DEBUG] Date: %s\n", date_text);
+#endif
+
+    sprintf(hour_text, "%02d", tm_now->tm_hour);
+    sprintf(minute_text, "%02d", tm_now->tm_min);
+    sprintf(second_text, "%02d", tm_now->tm_sec);
+#ifdef DEBUG
+    g_print("[DEBUG] Time: %s:%s:%s\n", hour_text, minute_text, second_text);
+#endif
+
+    gtk_entry_set_text(entry_date, date_text);
+    gtk_entry_set_text(entry_hour, hour_text);
+    gtk_entry_set_text(entry_minute, minute_text);
+    gtk_entry_set_text(entry_second, second_text);
+}
+
+static int gobjects_init(GtkBuilder *builder, GtkWidget *dialog) {
+    GObject *entry_date;
+    GObject *entry_hour;
+    GObject *entry_minute;
+    GObject *entry_second;
+    GObject *combo_hour;
+    GObject *combo_minute;
+    GObject *combo_second;
+    GObject *button_ok;
+    GObject *button_cancel;
+
+    gchar text[3] = {0};
+
+    entry_date = gtk_builder_get_object(builder, "entry-date");
+    if (entry_date == NULL) {
+        g_print("[ERROR] can not get entry date object\n");
+        return -1;
+    }
+
+    entry_hour = gtk_builder_get_object(builder, "comboboxtext-entry-hour");
+    if (entry_hour == NULL) {
+        g_print("[ERROR] can not get entry hour object\n");
+        return -1;
+    }
+
+    entry_minute = gtk_builder_get_object(builder, "comboboxtext-entry-minute");
+    if (entry_minute == NULL) {
+        g_print("[ERROR] can not get entry minute object\n");
+        return -1;
+    }
+
+    entry_second = gtk_builder_get_object(builder, "comboboxtext-entry-second");
+    if (entry_second == NULL) {
+        g_print("[ERROR] can not get entry second object\n");
+        return -1;
+    }
+
+    combo_hour = gtk_builder_get_object(builder, "comboboxtext-hour");
+    if (combo_hour == NULL) {
+        g_print("[ERROR] can not get combo box hour object\n");
+        return -1;
+    }
+    /* Add hour combo box context */
+    for (int i = 0; i < 24; i++) {
+        sprintf(text, "%d", i);
+        gtk_combo_box_text_append(GTK_COMBO_BOX_TEXT(combo_hour), text, text);
+    }
+
+    combo_minute = gtk_builder_get_object(builder, "comboboxtext-minute");
+    if (combo_minute == NULL) {
+        g_print("[ERROR] can not get combo box minute object\n");
+        return -1;
+    }
+    /* Add minute combo box context */
+    for (int i = 0; i < 60; i++) {
+        sprintf(text, "%d", i);
+        gtk_combo_box_text_append(GTK_COMBO_BOX_TEXT(combo_minute), text, text);
+    }
+
+    combo_second = gtk_builder_get_object(builder, "comboboxtext-second");
+    if (combo_second == NULL) {
+        g_print("[ERROR] can not get combo box second object\n");
+        return -1;
+    }
+    /* Add second combo box context */
+    for (int i = 0; i < 60; i++) {
+        sprintf(text, "%d", i);
+        gtk_combo_box_text_append(GTK_COMBO_BOX_TEXT(combo_second), text, text);
+    }
+
+    button_ok = gtk_builder_get_object(builder, "button-ok");
+    if (button_ok == NULL) {
+        g_print("[ERROR] can not get button ok object\n");
+        return -1;
+    }
+
+    button_cancel = gtk_builder_get_object(builder, "button-cancel");
+    if (button_cancel == NULL) {
+        g_print("[ERROR] can not get button cancel object\n");
+        return -1;
+    }
+
+    date_init(GTK_ENTRY(entry_date), GTK_ENTRY(entry_hour), GTK_ENTRY(entry_minute), GTK_ENTRY(entry_second));
+
+    g_signal_connect(entry_date, GTK_EVENT_ACTIVATE, G_CALLBACK(date_activate), "date");
+    g_signal_connect(button_ok, GTK_EVENT_CLICKED, G_CALLBACK(button_handler), "ok");
+    g_signal_connect(button_cancel, GTK_EVENT_CLICKED, G_CALLBACK(button_handler), "cancel");
+
+    return 0;
+}
 
 int main(int argc, char **argv) {
-    GtkApplication *app;
-    int status;
+    GtkBuilder *builder = gtk_builder_new();
+    GtkWidget *dialog;
+
+    gtk_init(&argc, &argv);
+
+    if (!gtk_builder_add_from_file(builder, "timemanager.ui", NULL)) {
+        g_print("[ERROR] can not load glade file\n");
+        return -1;
+    }
+
+    dialog = GTK_WIDGET(gtk_builder_get_object(builder, "dialog-date_time"));
+    if (dialog == NULL) {
+        g_print("[ERROR] can not get dialog object\n");
+        return -1;
+    }
+
+    gtk_window_set_title(GTK_WINDOW(dialog), "Date & Time");
+    gtk_window_set_default_size(GTK_WINDOW(dialog), 400, 200);
+
+    g_signal_connect(dialog, GTK_EVENT_DESTROY, G_CALLBACK(gtk_main_quit), NULL);
+
+    gobjects_init(builder, dialog);
+
+    get_time_date_status();
+    timezone_init(builder);
+
+    gtk_widget_show_all(dialog);
+    gtk_main();
+
+    return 0;
+
+//    GtkApplication *app;
+//    int status;
 
-    app = gtk_application_new("com.witium.timemanager", G_APPLICATION_FLAGS_NONE);
-    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
-    status = g_application_run(G_APPLICATION(app), argc, argv);
-    g_object_unref(app);
+//    app = gtk_application_new("com.witium.timemanager", G_APPLICATION_FLAGS_NONE);
+//    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
+//    status = g_application_run(G_APPLICATION(app), argc, argv);
+//    g_object_unref(app);
 
-    return status;
+//    return status;
 }

timemanager.ui

+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Generated with glade 3.18.3 -->
+<interface>
+  <requires lib="gtk+" version="3.12"/>
+  <object class="GtkDialog" id="dialog-date_time">
+    <property name="can_focus">False</property>
+    <property name="window_position">center</property>
+    <property name="type_hint">utility</property>
+    <property name="gravity">center</property>
+    <child internal-child="vbox">
+      <object class="GtkBox" id="dialog-vbox-date_time">
+        <property name="can_focus">False</property>
+        <property name="orientation">vertical</property>
+        <property name="spacing">2</property>
+        <child internal-child="action_area">
+          <object class="GtkButtonBox" id="dialog-action_area-date_time">
+            <property name="can_focus">False</property>
+            <property name="layout_style">end</property>
+            <child>
+              <object class="GtkButton" id="button-ok">
+                <property name="label" translatable="yes">OK</property>
+                <property name="visible">True</property>
+                <property name="can_focus">True</property>
+                <property name="receives_default">True</property>
+              </object>
+              <packing>
+                <property name="expand">True</property>
+                <property name="fill">True</property>
+                <property name="position">0</property>
+              </packing>
+            </child>
+            <child>
+              <object class="GtkButton" id="button-cancel">
+                <property name="label" translatable="yes">Cancel</property>
+                <property name="visible">True</property>
+                <property name="can_focus">True</property>
+                <property name="receives_default">True</property>
+              </object>
+              <packing>
+                <property name="expand">True</property>
+                <property name="fill">True</property>
+                <property name="position">1</property>
+              </packing>
+            </child>
+          </object>
+          <packing>
+            <property name="expand">False</property>
+            <property name="fill">False</property>
+            <property name="position">0</property>
+          </packing>
+        </child>
+        <child>
+          <object class="GtkBox" id="box-date_time">
+            <property name="visible">True</property>
+            <property name="can_focus">False</property>
+            <property name="orientation">vertical</property>
+            <child>
+              <object class="GtkBox" id="box-date">
+                <property name="visible">True</property>
+                <property name="can_focus">False</property>
+                <child>
+                  <object class="GtkLabel" id="label-date">
+                    <property name="visible">True</property>
+                    <property name="can_focus">False</property>
+                    <property name="label" translatable="yes">Date</property>
+                  </object>
+                  <packing>
+                    <property name="expand">True</property>
+                    <property name="fill">True</property>
+                    <property name="position">0</property>
+                  </packing>
+                </child>
+                <child>
+                  <placeholder/>
+                </child>
+                <child>
+                  <placeholder/>
+                </child>
+              </object>
+              <packing>
+                <property name="expand">True</property>
+                <property name="fill">True</property>
+                <property name="position">0</property>
+              </packing>
+            </child>
+            <child>
+              <object class="GtkEntry" id="entry-date">
+                <property name="visible">True</property>
+                <property name="can_focus">True</property>
+                <property name="margin_left">48</property>
+                <property name="margin_right">48</property>
+              </object>
+              <packing>
+                <property name="expand">True</property>
+                <property name="fill">False</property>
+                <property name="padding">2</property>
+                <property name="position">1</property>
+              </packing>
+            </child>
+            <child>
+              <object class="GtkBox" id="box-time">
+                <property name="visible">True</property>
+                <property name="can_focus">False</property>
+                <child>
+                  <object class="GtkLabel" id="label-time">
+                    <property name="visible">True</property>
+                    <property name="can_focus">False</property>
+                    <property name="label" translatable="yes">Time</property>
+                  </object>
+                  <packing>
+                    <property name="expand">True</property>
+                    <property name="fill">True</property>
+                    <property name="position">0</property>
+                  </packing>
+                </child>
+                <child>
+                  <placeholder/>
+                </child>
+                <child>
+                  <placeholder/>
+                </child>
+              </object>
+              <packing>
+                <property name="expand">True</property>
+                <property name="fill">True</property>
+                <property name="position">2</property>
+              </packing>
+            </child>
+            <child>
+              <object class="GtkBox" id="box-time_clock">
+                <property name="visible">True</property>
+                <property name="can_focus">False</property>
+                <child>
+                  <object class="GtkComboBoxText" id="comboboxtext-hour">
+                    <property name="visible">True</property>
+                    <property name="can_focus">False</property>
+                    <property name="button_sensitivity">on</property>
+                    <property name="has_entry">True</property>
+                    <child internal-child="entry">
+                      <object class="GtkEntry" id="comboboxtext-entry-hour">
+                        <property name="can_focus">False</property>
+                      </object>
+                    </child>
+                  </object>
+                  <packing>
+                    <property name="expand">True</property>
+                    <property name="fill">True</property>
+                    <property name="padding">2</property>
+                    <property name="position">0</property>
+                  </packing>
+                </child>
+                <child>
+                  <object class="GtkComboBoxText" id="comboboxtext-minute">
+                    <property name="visible">True</property>
+                    <property name="can_focus">False</property>
+                    <property name="button_sensitivity">on</property>
+                    <property name="has_entry">True</property>
+                    <child internal-child="entry">
+                      <object class="GtkEntry" id="comboboxtext-entry-minute">
+                        <property name="can_focus">False</property>
+                      </object>
+                    </child>
+                  </object>
+                  <packing>
+                    <property name="expand">True</property>
+                    <property name="fill">True</property>
+                    <property name="padding">2</property>
+                    <property name="position">1</property>
+                  </packing>
+                </child>
+                <child>
+                  <object class="GtkComboBoxText" id="comboboxtext-second">
+                    <property name="visible">True</property>
+                    <property name="can_focus">False</property>
+                    <property name="button_sensitivity">on</property>
+                    <property name="has_entry">True</property>
+                    <child internal-child="entry">
+                      <object class="GtkEntry" id="comboboxtext-entry-second">
+                        <property name="can_focus">False</property>
+                      </object>
+                    </child>
+                  </object>
+                  <packing>
+                    <property name="expand">True</property>
+                    <property name="fill">True</property>
+                    <property name="padding">2</property>
+                    <property name="position">2</property>
+                  </packing>
+                </child>
+              </object>
+              <packing>
+                <property name="expand">True</property>
+                <property name="fill">True</property>
+                <property name="position">3</property>
+              </packing>
+            </child>
+            <child>
+              <object class="GtkBox" id="box-timezone">
+                <property name="visible">True</property>
+                <property name="can_focus">False</property>
+                <child>
+                  <object class="GtkLabel" id="label-timezone">
+                    <property name="visible">True</property>
+                    <property name="can_focus">False</property>
+                    <property name="label" translatable="yes">TimeZone</property>
+                  </object>
+                  <packing>
+                    <property name="expand">True</property>
+                    <property name="fill">True</property>
+                    <property name="position">0</property>
+                  </packing>
+                </child>
+                <child>
+                  <placeholder/>
+                </child>
+                <child>
+                  <placeholder/>
+                </child>
+              </object>
+              <packing>
+                <property name="expand">True</property>
+                <property name="fill">True</property>
+                <property name="position">4</property>
+              </packing>
+            </child>
+            <child>
+              <object class="GtkComboBox" id="combobox-timezone">
+                <property name="visible">True</property>
+                <property name="can_focus">False</property>
+                <property name="margin_left">48</property>
+                <property name="margin_right">48</property>
+                <property name="active">0</property>
+              </object>
+              <packing>
+                <property name="expand">True</property>
+                <property name="fill">False</property>
+                <property name="padding">2</property>
+                <property name="position">5</property>
+              </packing>
+            </child>
+          </object>
+          <packing>
+            <property name="expand">True</property>
+            <property name="fill">True</property>
+            <property name="position">1</property>
+          </packing>
+        </child>
+      </object>
+    </child>
+  </object>
+</interface>