OVERLAPS for Informix
I don't think there is a OVERLAPS() function that you could use. You'll have to stick to the usual test:

WHERE a.StartTime &lt; b.EndTime
  AND b.StartTime &lt; a.EndTime


Change &lt; to &lt;=, depending on how you want your Overlap test to function on end cases, when an end time (or date) is exactly the same as the other start time. I guess such behaviour may be wanted for dates.