Datetime format started_at and ended_at PHP problem

My request : ‘https://api.twitch.tv/helix/clips?game_id=’.$game_id.’&started_at=’.$started_at.’&ended_at=’.$ended_at.’&first=100&language=’.$lang;

I think it’s the format of the date that is not good, I try two formats but it still does not work.

Exemple with : echo date(DATE_RFC3339);
Output : 2018-10-04T22:21:59+02:00

I think Twitch not like “+02:00” ?