GridView validator rejects value previously cleared when row is edited a second time

Hello,
I am using a gridview with a start date and an end date. I enter the start date and the range validator specified in the edit template field for the start date correctly handles the entered value.
However, when returning to the the data 'later' to enter the end date, the range validator for the Start date (still containing a legal date from the prior update) now flags the start date as an illegal value.
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