Use of "possibly" vs. "possible"?

Why possibly and not possible? What is the adverb "possibly" modifying in this sentence?

Setting a cut-off date, possibly sometime in the first half of 2018, would aim to protect plants vulnerable to dicamba.


Per dictionary.com possibly means "perhaps, maybe". So that phrase means "perhaps sometime in the first half of 2018".

In order to use possible you'd have to write something like:

It's possible we would set the cutoff date sometime in the first half of 2018.