For all I know, Batch does not have a command that gives the UNIX time. The closest one I can find is %time%, which only displays the timestamp.
Is ther
By far best solution is to download a freestanding date.exe unix-port.
Recommend that you rename it to unixdate.exe, to avoid conflict with MS Date command.
Get it from here
Example:
((PROMPT)):unixdate +%Y%M%d
20141704
((PROMPT)):unixdate +%Y%b%d
2014Sep04