Compare and validate two date fields

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ASP.NET

CSS

C#5.0

hi to all,
I want to compare and validate two date fields in asp.net form. but here i done compare two date fields in same tab is finished.but i want to compare two date fields with different tabs.

In my module i have two tabs like customer ,sales

I compare one date field in customer, and another date field in sales .how to do this task.