360 is the number of degrees in a circle, usually representing a complete, or 'whole' turn (hence "a whole 360").
There are a number of commonly used phrases that include '360' to denote a complete turn around.
Actually, I feel that a lot of such phrases are technically wrong. For example, I've seen "360 degree turn" used to mean a change in direction, similar to the phrase "a U-turn". But a turn in the opposite direction would only be 180 degrees - turning 360 degrees means you would end up facing the same direction you began with.
Given the context of your quote - moving a date forward - this seems like another example of it being an ill-fitting use of the term. That could be intentional on the part of the writers - scripts use everyday language, even if terms are being used incorrectly. It seems that the character Fish Bang is referring to Jimmy changing his mind.