How to exclude rows with no value or blank from column count

Hi everyone

i'm getting started with power bi and i'm having some trouble, i have been requested to count all workers that have worked during a month (none specific) and not all workers have started at the same date therefore only some of them have been working the entire month since the other workers entered later in the month some of them don't register activity before that date (logically)

I have a table with the name of the worker, and some columns that i have pivoted that contain a determined character in regard of what they did in the month as well as the date when they did such activity, the column of activity has values such as w for worked, r for rest, f for fired and so on.

what i need to do is have the exact number of workers that are up to certain date, i have the date filter working good so that is not the problem, what i need power bi to do is to ignore the workers that for example started on JAN 10 to appear in the countdowns before that date, i'm trying to count blank spaces in activity but doesn't work (since they didn't had before), and also i tried to count rows that are not blank and didn't work either, how can i do this? i'd be ready to answer any doubt regarding my question, thanks in advance