To get the last date of the previous month, build a LastMonth calculated field (Date/Time type) with this expression:
I have monthly reports that do not contain a date in them anywhere. /quote
It is very rare, in my experience, for someone to write a report and not include some sort of usable reference somewhere, if only for audit trail purposes.
If you have a meaningful date in, say, the page header area, selection criteria or, at a stretch, even from the report file created date (though who knows how meaningful such a date might be ...) there should be some possibility to use it one way or another along the lines Kruncher and Ralph mention above.
Failing that (and with my tongue deep in my cheek) you could randomly select any date since no one is likely to know any better!