¿How to create a validation with efective start date in pages with When and Why section?

Summary:

I want to create a validation with effective start date but I can't find that field exactly.

Content (please ensure you mask any confidential information):

I want to create a validation with effective start date but I can't find that field exactly or I don´t know if I'm looking for in the wrong way. I want to create a rule to validate that date and depending of the date that the user select, compare against another defined dates and shows a message if that conditions is meet. ¿Do you know if that filed exist or what's correct name ?