Question about checking one field compared to another one from a different table

Hello Experts,

 

I have a quick question to ask or a task to ask about.

If you have a field from one table and need to compare it to a field from another table, what is the method of doing this?

For example, if I wanted to compare the time that an approver(approval table/log) approves an RFC to a time/date field in the cm3r table). How would that be done?

The task that I have been presented with is that if an approver tries to approve an RFC that is past the implementation time.

Do I check during the Authorization phase? Do I check the time/date fields and have a rule set to lock out the change?

Not sure. 

Thanks for your help with this issue.