Yes, i am able to reproduce the problem in Linux but not in windows. I am attaching the fix for this issue. After applying this fix, the behaviour in windows and linux are same.
Could you confirm that, are you facing the issue with the debug version of pgAdmin. With the installed pgAdmin, i too facing the problem but the date is setting to 1970. But the debug pgAdmin is doing fine in my windows 7.
I noticed one other issue in the role dialogue that I'd like you or Dhiraj to look at please. On my Mac, if I open the properties for a role with no VALID UNTIL date set, it's setting the date in the dialogue to the current date and generating the SQL to make that change. It should leave the date empty of course, and not generate any SQL until the user actually changes something.