dateDiff Function does not work

Hi all,

I am experiencing an issue with the dateDiff function when trying to find the time between two dates in hours. So I have two fields, a time in and time out, these fields are set to validation type "time". I then have a field that looks like this: dateDiff("h", mon_lunch_start, mon_lunch_end)

If the time in mon_lunch_start is set at 13:00 or anytime after then the function will display the correct number of hours between mon_lunch_start and mon_lunch_end, for instance if mon_lunch_start=13:00 and mon_lunch_end=15:00 the dateDiff will return "2".

However if mon_lunch_start is set at any time before 13:00 then dateDiff will return an arbitrary number in the thousands. For example; if mon_lunch_start=12:30 and mon_lunch_end=13:30 then the dateDiff will return 69382. Changing it to 12:31 will then return another random number.

Is this function broken? Does anyone have any experience with this?