By "backdating" they will go back to the most recent (sometimes second most recent) challenge start date (which is the 1st and 15th of the month).

The agent probably said they can't backdate if you were asking them to go all the way back to December to count your miles. Miles before you started the challege "don't count" towards the value needed in the challege to get your status.

Did I just confuse the situation even more??

So, start counting your miles for any flights on or after April 16th....you have until July 16th. Good luck!