@Scott: Technically speaking, INTCK function is *not* appropriate for calculating the duration between two time points, especially the unit of time you use is larger than the accuracy of the time points you have. For example, I don't think it is appropriate to measure the duration between two datetimes (measured at the second unit) using a function call like INTCK('day',...). The reason being that the INTCK function enumerates the *boundaries* in the given interval. This means that in terms of 'DAY' interval (without any shifts), the INTCK counts how many midnights(00:00am) there are between the two given time points, which may be somewhat different from the counts of how many 24-hours between the two given time points.
In this case, OP is clear about what s/he wants -- the duration --, thus I would simple calculate it by subtracting the start from the finish (this will give the duration in terms of seconds). If the OP is asked to report it in other time units(minutes, hours, days), then a simple division/multiplication will do.