As far as I can tell, because Date::Manip uses it's own/ISO representation internally for dates, times and delta's, Gregorian -v- Julian -v- other calander systems aside, it should do the right thing for pretty much any set of dates that you want to throw at it. I haven't personnally verified this, but the module's been around long enough that any such fopars would probably have shown up by now.

As for that code that I posted (besides being bug-ridden), it's just long strings of 1's and 0's, so there shouldn't be any problems as your only representing days. However it may not accurately account for leap days as you can't represent 0.2439 (from memory) days in a bit or byte. Meaning that if you need the kind of accuracy that would account for this, the method may not be useful to you. People joining or leaving on Feb 29th, or counts of active members on ranges that start or finish on Feb 29th could be mis-counted. I can't see an easy fix for this.

The only other comment is that with a range of 50+ years and 4500+ members, the storage requirements rise to close to 80 MB. Moving to using a bit/day instead of byte/day this would reduce to 10MB which might be worth the effort, but vec isn't a direct substitute for substr unfortunately, so the code gets more complicated.