How do I assign a start date?

[#1]

I have a longitudinal study with sites entering on different dates. To make them comparable, I need to assign each entry date as "1" and the second data collection point as "2" and so on. The data are in long format and there are a lot of missing data (missed data collection)

The data look like this, I want to generate "Count":

ID    Date   Var1  Count

001  Jan-1   .          .

001  Feb-1   .          .

001  Mar-1   10       1

001  Apr-1    13       2

002  Jan-1    8         1

002  Feb-1   10        2

002  Mar-1   .           .

I used a JRP formula, but it counts every line whether the first measure is missing or not.

[#2]

Thanks!
I've been trying to learn JMP programming and this interaction helps, especially when odd requests aren't in the books.