DateTime.Now vs. DateTime.UtcNow

One main concept to understand in .NET is that now is now all over the earth no matter what time zone you are in. So if you load a variable with DateTime.Now or DateTime.UtcNow -- the assignment is identical.* Your DateTime object knows what timezone you are in and takes that into account regardless of the assignment.

The usefulness of DateTime.UtcNow comes in handy when calculating dates across Daylight Savings Time boundaries. That is, in places that participate in daylight savings time, sometimes there are 25 hours from noon to noon the following day, and sometimes there are 23 hours between noon and noon the following day. If you want to correctly determine the number of hours from time A and time B, you need to first translate each to their UTC equivalents before calculating the TimeSpan.

This is covered by a blog post i wrote that further explains TimeSpan, and includes a link to an even more extensive MS article on the topic.

*Clarification: Either assignment will store the current time. If you were to load two variables one via DateTime.Now() and the other via DateTime.UtcNow() the TimeSpan difference between the two would be milliseconds, not hours assuming you are in a timezone hours away from GMT. As noted below, printing out their String values would display different strings.

Also note the performance difference; DateTime.UtcNow is somewhere around 30 times faster then DateTime.Now, because internally DateTime.Now is doing a lot of timezone adjustments (you can easily verify this with Reflector).

So do NOT use DateTime.Now for relative time measurements.

DateTime.UtcNow tells you the date and time as it would be in Coordinated Universal Time, which is also called the Greenwich Mean Time time zone - basically like it would be if you were in London England, but not during the summer. DateTime.Now gives the date and time as it would appear to someone in your current locale.

I'd recommend using DateTime.Now whenever you're displaying a date to a human being - that way they're comfortable with the value they see - it's something that they can easily compare to what they see on their watch or clock. Use DateTime.UtcNow when you want to store dates or use them for later calculations that way (in a client-server model) your calculations don't become confused by clients in different time zones from your server or from each other.

It's really quite simple, so I think it depends what your audience is and where they live.

If you don't use Utc, you must know the timezone of the person you're displaying dates and times to -- otherwise you will tell them something happened at 3 PM in system or server time, when it really happened at 5 PM where they happen to live.

We use DateTime.UtcNow because we have a global web audience, and because I'd prefer not to nag every user to fill out a form indicating what timezone they live in.

We also display relative times (2 hours ago, 1 day ago, etc) until the post ages enough that the time is "the same" no matter where on Earth you live.