Which WebControl are you using? Did you try?
DateTime.Now.ToString("yyyy-MM-dd");
The ToString method on the DateTime struct can take a format parameter:
ToString
DateTime
var dateAsString = DateTime.Now.ToString("yyyy-MM-dd");// dateAsString = "2011-02-17"
Documentation for standard and custom format strings is available on MSDN.
<%= DateTime.Now.ToString("yyyy-MM-dd") %>