You can try something like:
IF DATEPART('hour', ATTR([Your Date])) >= 15 and datepart('minute', ATTR([Your Date])) >= 30
THEN count([Employee Number]) END
A lot also depends on how you have your view set up. So if this doesn't work, some screenshots or even better a sample workbook would help.
Thanks Pooja, I had to alter it slightly for my purpose but it worked great.