form validation to compare two dates

[eluser]veledrom[/eluser]
Hi,

I want to compare two dates by using form_validation().

<b>First submission</b> must be earlier than <b>Last submission</b> date.

Date format is coming as <b>dd/mm/yyyy</b> from the web form.
I read the callbacks but unfortunately I couldn't do it.

How can I do it?