I should specify that I do start out with two dates to generate the period - thinking about it, you can't say exactly how many days are in a month. So I guess what I'm actually looking for is the number of days between two dates

Have you tried a combination of between and divide?

(tick/divide (tick/between <some instant> <some other instant>)
	         (tick/new-duration 1 :days))

This would work for durations (between gives a duration) but not for periods, for the reasons you've just given.

I did have a look at divide - but as you say I was using periods and I was getting some thing like ‘doesn’t work on estimates’. It sounds like converting the dates I was using to instants before calling between would solve that. I’ll give it a go, thanks!

Thanks!