Why does the January 200 cycling challenge only start half way through the month, rather than last all month..? Can it be backdated so the first two weeks actually count?

It is the way that type of challenge is setup. It isn't a month cycling challenge. It is "Ride to 200" challenge. They are only for two weeks. "Ride to 100" are for one week. They are usually not starting on the 1st of the month. I think there has only been one "Ride to 200" challenge that has started the 1st and that was in August. All others are starting in the middle or late in the month.

It can't be backdated.