Yes, the challenge deadline depends on the challenge you choose. Monthly accumulator challenges end on the last day of each challenge month.
All distance challenges end on a Sunday. Depending on which distance you choose, your challenge period will be 7 or 28 days. If you are doing a 7 day challenge then the finish deadline is before midnight on Sunday. If you are doing a 28 day challenge then the finish deadline is before midnight on the fourth Sunday.
Challenge results can be submitted on the challenge page up to 7 days after the finish deadline of your challenge. If results are not submitted within 7 days we will assume the challenge was not completed and you will not receive a medal.