How a magic item that recharges at dawn is on a plane without a dawn?

@JeremyECrawford A staff of healing recharges at dawn. What if the party is on a plane without a dawn?

— Jeremy Land (@WatchtowerNorth) February 23, 2016

If you're on a plane of existence without a dawn, your magic item recharges at a dawn-equivalent time. #DnD https://t.co/pBrOJtugwJ

— Jeremy Crawford (@JeremyECrawford) February 27, 2016