Hope you had a lovely Christmas and New year celebrations

I just have an below urgent query and need your help on it. Request you to provide your valuable inputs which really helps my situation.
Visa Type: Tier 2 General Extension
End date mentioned in COS : 01/02/2017
End date mentioned on BRP card : 15/02/2017
Current status: visa switching is under process
Query: Until what date can I work for my current employer (01/02 or 15/02) even after new visa
stamped?
Please help guys!! Many thanks in advance.