Convert MJD date range to number of days elapsed in IDL
Like @James K said, the MJD is simply a count, so you can set some arbitrary date to be 0 and go from there. 

For example, take today's MJD date and say there was one event today:

Today = 57657 


Add three months and 12 days to the next event:

Today + 103 = 57760


So if you called ten days before the first event 0, then the "dates" of the first two events would simply be the following:

Start = Event_1 - 10 = 0 
Event_1 = Start + 10 = 10 &lt;-- Semantics right!
Event_2 = Event_1 + 103 = 113


It really is that simple. Just set a variable to equal whatever starting MJD you want to be zero and count earth days between events.