Script vs. if() depends on whether you want the numbers to stay the same or change as you make selections. Like if the dates are two years apart, but other selections can make it so that you're only interested in one month of those two years, do you want to count days in the month or days in the two years? If the month, use if(). If two years, do it in script.
I think the if() would look something like this, assuming you have a calendar with a Date field.
count({<Status={'5'}>} distinct if(Date>=CreatedOn and Date<=ResolvedOn,Date)))
A more complicated option, if you want the results of the count(if()) instead of a fixed script solution, but need it to perform, is probably to handle it in the data model. I suppose that's still a script solution but different than actually counting in the script. Something like this, maybe, though it's hard to guess without knowing more about your data model.
Data:
ID, CreatedOn, ResolvedOn
A, January 4, January 7
B, January 5, January 6
NewTable:
ID, Date
A, January 4
A, January 5
A, January 6
A, January 7
B, January 5
B, January 6
count({<Status={'5'}>} distinct Date)
