How reliable is DateTime.UtcNow in Silverlight applications?

Question

I have a silverlight application which users will be running in various time zones.

These applications load their data from the server upon start up, then cache it in IsolatedStorage.

When I make changes to the data on the server, I want to be able to change the "last updated time" so that all silverlight clients download the newest data the next time they check this date.

However, I'm a bit confused as to how to handle the time zone issue since a if the server is in New York and the update time is set to 2010-01-01 17:00:00 and a client in Seattle checks compares it to its local time of 2010-01-01 14:00:00 it won't update and will continue to provide old data for three more hours.

My solution is to always post the update time in UTC time, not with the time on the server, then make the Silverlight app check with DateTime.UtcNow.

Is this as easy as it sounds or are their issues with this, e.g. that timezones are not set correctly on computers and hence the SilverlightApp does not report the correct UTC time. Can anyone say from experience how likely it is that using DateTime.UtcNow like this for cache refreshing will work in all cases?

If DateTime.UtcNow is not reliable, I will just use an incremented "DataVersion" integer but there are other scenarios in which getting time zone sychronization down would make it useful to thoroughly understand how to solve this in silverlight apps.

Answer 1

DateTime.UtcNow is as reliable as the clock on the client system. So the question is entirely independent of Silverlight or .NET, the question is how much do you trust the system clock on the client machines?

You need to weigh the risk that a user of a machine may have incorrectly set the time on their machine because they have not set the time zone correctly. This risk is entirely human in nature.

Using an incrementing version number only has one downside, you need to first retrieve the current value before you can set a new one. If that isn't a problem then go with that and eliminate the FUD you might have around time zones.