Not sure if this the right answer or there's a proof for it but I assume it's the same time they were originally got sent back from, which is 2027 and in this time line, Alison still alive and no one know John
Here's Derek wikia, he was born in 1995 and he's 32 years old, which means that he was sent back from 2027.
and from Alison wikia
Following the deactivation of Cameron at the hands of John Henry, John Connor and Catherine Weaver jumped to 2027. However, this was an alternate timeline where John Connor never became leader of the Resistance. There they found Allison Young alive and well with Derek and Kyle Rees.
Edit : OK, That was not really correct. From this
Q: What year did John Connor and Catherine Weaver jump to?
A: Unknown. There was no display indicating the destined time period in John Henry's chamber, only a countdown clock. There is no official date given for their arrival in the episode; however, it could be assumed that it is either 2026 or 2027 by the ages of Kyle and Derek. We know only that it is sometime after Judgment Day. It could be, or may not be, before John Connor takes control over the Resistance. In this new timeline, it is possible John may not have existed (which is likely due to the transport as no one would have grown up to become John Connor).