On this example marks management will be explained. Functions elm_calendar_mark_add(), elm_calendar_mark_del() and elm_calendar_marks_clear() will be covered.

To add a mark, will be required to choose three things:

mark style
mark date, or start date if it will be repeated
mark periodicity

Style defines the kind of mark will be displayed over marked day, on calendar. Default theme supports holiday and checked. If more is required, is possible to set a new theme to calendar widget using elm_object_style_set(), and use the signal that will be used by such marks.

Date is a struct tm , as defined by time.h. More can be read on ctime manpage. If a date relative from current is required, this struct can be set as:

   current_time = time(NULL);
   localtime_r(&current_time, &selected_time);

Or if it's an absolute date, you can just declare the struct like:

   struct tm sunday;
   struct tm christmas;

   /*
    * At least on Windows, tm has 9 fields.
    * As a workaround, set sunday to 0 and set
    * th needed fields to correct value
    */
   memset(&sunday, 0, sizeof(struct tm));
   sunday.tm_hour = 12;
   sunday.tm_mday = 7;
   sunday.tm_isdst = -1;

   memset(&christmas, 0, sizeof(struct tm));
   christmas.tm_mday = 25;
   christmas.tm_mon = 11;

Periodicity is how frequently the mark will be displayed over the calendar. Can be a unique mark (that don't repeat), or it can repeat daily, weekly, monthly or annually. It's enumerated by Elm_Calendar_Mark_Repeat_Type.

So let's add some marks to our calendar. We will add christmas holiday, set Sundays as holidays, and check current day and day after that.

   struct tm sunday;
   struct tm christmas;

   /*
    * At least on Windows, tm has 9 fields.
    * As a workaround, set sunday to 0 and set
    * th needed fields to correct value
    */
   memset(&sunday, 0, sizeof(struct tm));
   sunday.tm_hour = 12;
   sunday.tm_mday = 7;
   sunday.tm_isdst = -1;

   memset(&christmas, 0, sizeof(struct tm));
   christmas.tm_mday = 25;
   christmas.tm_mon = 11;
   current_time = time(NULL);
   localtime_r(&current_time, &selected_time);
   mark = elm_calendar_mark_add(cal, "checked", &selected_time,
                                ELM_CALENDAR_UNIQUE);

   /* check tomorrow */
   current_time = time(NULL) + 1 * SECS_DAY;
   localtime_r(&current_time, &selected_time);
   elm_calendar_mark_add(cal, "checked", &selected_time, ELM_CALENDAR_UNIQUE);

   /* mark christmas as holiday */
   elm_calendar_mark_add(cal, "holiday", &christmas, ELM_CALENDAR_ANNUALLY);

   /* mark Sundays as holidays */
   elm_calendar_mark_add(cal, "holiday", &sunday, ELM_CALENDAR_WEEKLY);

We kept the return of first mark add, because we don't really won't it to be checked, so let's remove it:

   elm_calendar_mark_del(mark);

After all marks are added and removed, is required to draw them:

   elm_calendar_marks_draw(cal);

Finally, to clear all marks, let's set a callback for our button:

   bt = elm_button_add(win);
   evas_object_size_hint_weight_set(bt, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   evas_object_size_hint_align_set(bt, EVAS_HINT_FILL, EVAS_HINT_FILL);
   elm_object_text_set(bt, "Clear marks");
   evas_object_smart_callback_add(bt, "clicked", _btn_clear_cb, cal);
   elm_box_pack_end(bx, bt);
   evas_object_show(bt);

This callback will receive our calendar object, and should clear it:

static void
_btn_clear_cb(void *data, Evas_Object *btn EINA_UNUSED, void *ev EINA_UNUSED)
{
   Evas_Object *cal = data;
   elm_calendar_marks_clear(cal);
   elm_calendar_marks_draw(cal);
}

Note:: Remember to draw marks after clear the calendar.

See the full source code calendar_example_06::c here.