Thanks for the response.

I can imagine a constant current source.
I can imagine starting it, and stopping it with a comparator.
I can't imagine how to time the interval in PIC basic. The books I have don't help, or I am not looking in the right place. (I am just a retired bookkeeper having fun with this.)

I don't want to get involved with a separate counter. The author who got me started on this ran into all kinds of stability problems with his on the small caps.