I seem to recall others have asked this type of question, especially on the Automation Direct forum, but I didn't feel like looking, so I created this little snippet that works on a D0-06 and should work on most any DirectLogic processor since it is only a 4 digit BCD timer. If you are using something else, like an 8 digit timer the instructions will need tweaking for doubles.
I didn't use any Iboxes, mostly because they "hide" the memory locations in the equation from the cross reference, at least in DS5 and before. I don't yet have DS6 (because I have Do-more!) Also, the check for a negative subtraction result is easier to do this way, at least to me.
(Yes franji1 and BobO, I know it isn't "hiding" it really, but the result is the same.)