Should I use DateTime.Now or DateTime.UtcNow in HttpCookie.Expires and HttpCachePolicy.SetExpires?

It doesn't matter in this case.

Internally, the first thing .SetExpires does is convert your supplied datetime into UTC, before setting it on the cookie.

Bear in mind, as long as your datetime consumer uses the DateTime class correctly, then the two are the same - it is just that one is "baselined" to UTC and the other isn't:

20110701T14:00:00-1:00 (British Summer Time)

and

20110701T13:00:00+0:00 (UTC)

represent exactly the same datetime, namely 1pm UTC.

As long as the consumer handles this correctly (which it seems to, having looked in reflector) then it makes no difference.

If you were taking this and passing it in as a time string, then of course, it may well make a difference, but not in this case.

You can see the effect with the following code (assuming you are not in UTC yourself - if you are - change your settings to test!). They both output the same datetime, once you've asked for it to be converted to UTC.

WriteDateTime(DateTime.Now);WriteDateTime(DateTime.UtcNow);public static void WriteDateTime(DateTime dateTime){   Console.WriteLine(dateTime.ToUniversalTime().ToLongTimeString());   }

You should be using DateTime.UtcNow method because thats the time standard used for cookies. UTC is equivilant to GMT.

From MSDN: System.DateTime.UtcNow

Gets a DateTime object that is set to the current date and time on this computer, expressed as the Coordinated Universal Time (UTC).

Refer to this for an explanation between them.