GladeTime

#include <gtk/gtk.h>

typedef struct {
    GtkWidget *w_lbl_time;
} app_widgets;

gboolean timer_handler(app_widgets *widgets);

int main(int argc, char *argv[])
{
    GtkBuilder      *builder;
    GtkWidget       *window;
    app_widgets        *widgets = g_slice_new(app_widgets);

    gtk_init(&argc, &argv);

    builder = gtk_builder_new();
    gtk_builder_add_from_file (builder, "glade/window_main.glade", NULL);

    window = GTK_WIDGET(gtk_builder_get_object(builder, "window_main"));
    widgets->w_lbl_time  = GTK_WIDGET(gtk_builder_get_object(builder, "lbl_time"));
    gtk_builder_connect_signals(builder, widgets);
    g_object_unref(builder);

    // start 1 second timer
    g_timeout_add_seconds(1, (GSourceFunc)timer_handler, widgets);

    gtk_widget_show(window);
    gtk_main();
    g_slice_free(app_widgets, widgets);

    return 0;
}

// handler for the 1 second timer tick
gboolean timer_handler(app_widgets *widgets)
{
    GDateTime *date_time;
    gchar *dt_format;

    date_time = g_date_time_new_now_local();                        // get local time
    dt_format = g_date_time_format(date_time, "%H:%M:%S");            // 24hr time format
    gtk_label_set_text(GTK_LABEL(widgets->w_lbl_time), dt_format);    // update label
printf("%s\n", dt_format);
    g_free (dt_format);

    return TRUE;
}

// called when window is closed
void on_window_main_destroy()
{
    gtk_main_quit();
}

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.18.3 -->
<interface>
  <requires lib="gtk+" version="3.12"/>
  <object class="GtkWindow" id="window_main">
    <property name="can_focus">False</property>
    <property name="title" translatable="yes">Time</property>
    <signal name="destroy" handler="on_window_main_destroy" swapped="no"/>
    <child>
      <object class="GtkBox" id="box1">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="margin_left">10</property>
        <property name="margin_right">10</property>
        <property name="margin_top">10</property>
        <property name="margin_bottom">10</property>
        <property name="spacing">7</property>
        <property name="homogeneous">True</property>
        <child>
          <object class="GtkLabel" id="label1">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="label" translatable="yes">Current Time:</property>
            <attributes>
              <attribute name="weight" value="bold"/>
            </attributes>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">0</property>
          </packing>
        </child>
        <child>
          <object class="GtkLabel" id="lbl_time">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="label" translatable="yes">Time</property>
            <attributes>
              <attribute name="weight" value="bold"/>
            </attributes>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">1</property>
          </packing>
        </child>
      </object>
    </child>
  </object>
</interface>