There may be an easier way to do this...

I am trying to see if the user is logged in between 5:15 PM today and 1:55 AM tomorrow

Code:
If DATEDIFF("N",FormatDateTime(Date & " 5:15 PM", 0),FormatDateTime(Date & " " & Time, 0)) > 0 AND & _
DATEDIFF("N",FormatDateTIme(DATEADD("D",1, Date) & " 1:55 AM", 0),FormatDateTime(Date & " " & Time, 0)) < 0 Then
Returns 'True' if between the two times and dates specified