Compare Two dates and find which one is smaller in TextBox in ASP.Net

i have two textbox as naming : txtInitiateDt and txtDisposalDt.

when user give 2nd textbox value from datepicker it should chk whether the given value shouldnt more than to date and also less that 1st textbox value.

i did it & also works good...bt there's an error arise when i host the site on client machine where only framework and database is present.

and the error is "String Value is not in correct dateformat" and fr this exception other events are blocked.