Range validator is not working properly with dates

I have a date range validator that fails about 3% of the hits. While the rest 97% of the hits this validation works fine.

This error message is :" The value '' of the MaximumValue property of 'vldDateRange' cannot be converted to type 'Date' ".
Please note the following:
The Range validator type is set to Date.The MaximumValue is always set on Page load using system time as: vldDateRange.MaximumValue = DateTime.Today.Date.ToString("dd/MM/yyyy");
The MinimumValue is always set on page load too, but never failed, and is set as: vldDateRange.MinimumValue = DateTime.Today.AddDays(-5).ToString("dd/MM/yyyy");
System globalization is set on server also using globalization in Web.config file:
<globalization culture="en-GB" uiCulture="e ...