Compare validator /Range validator does not work while using localization

Hi,
I have a requirement of converting existing english form to multi lingual form.I am currently converting english form to spanish form.
Everything works except the date formats.
The range validator is not working in a spanish form but works in a english form.
I have set the maximum and minimum value in page load as below in the english form
Heres how i have set the values
rndateval.MinimumValue = DateTime.Now.AddYears(-50).ToShortDateString();
rndateval.MaximumValue = DateTime.Now.AddDays(20).ToShortDateString();

But the range validator does not work
Please help me in resolving the issue .
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