Using Coordinated Universal Time (UTC) to Store Date/Time Values
By Scott Mitchell
Introduction
Many data-driven applications have date/time values that are automatically populated with the current date and time. For
example, in an online messageboard, whenever a user makes a post the current date and time is recorded and saved with that
post. Another example is in applications that track created on or last modified dates. For these types of applications,
whenever a new record is created or an existing one modified, the current date and time are written into these respective
fields.
Such auto-entered timestamp values can be stored in the database in either server time or
Coordinated Universal Time (UTC). If you use SQL Server's built-in getdate()
function or DateTime.Now property
to determine the current time, then you are using server time and saving the date and time respective to the web or
database server's time zone settings. Alternatively, if you use SQL Server's built-in
getutcdate() function
or DateTime.UtcNow property
then you are storing the values in UTC.
There are a number of advantages in storing such date/time values in UTC rather than server time. In this article we will
explore those advantages as well as look at a simple sample application that illustrates how to save dates in UTC and
still display them relative to the time zone's offset.
What is UTC?
UTC, or Coordinated Universal Time, is the standard
international time that all time zones are expressed as offsets of. UTC does not get adjusted for daylight savings.
To compute local time from UTC, simply add the time zone offset and then add an additional hour if daylight savings time is
in effect.
As I write this sentence, the time in UTC is Friday, August 3, 2007 18:18:36. California has a UTC offset of -8 and is
in daylight savings time. Therefore my local time is Friday, August 3, 2007 11:18:36. To compute the local time I simply
subtracted eight hours then added an hour back in for daylights savings.
Why and When You Should Use UTC
UTC should be used in situations when date/time values are automatically entered by the system to record when a record
was added or last updated. For example, in an eCommerce website, when an order was placed you'd want to record the
date and time the order was made.
The primary advantage of storing date/time values in UTC is that it makes the data transportable. To see what I mean, imagine
that following scenario: you have an eCommerce website that is being hosted in a web server located in
the Pacific time zone (UTC -8) and this application stores the date and time orders were placed in server time.
Say a user, Bob, makes an order on August 1, 2007 at 9:00 AM UTC -8. After many months of phenomenal growth, you decide to switch to a
larger web hosting company, one on the east coast where the time zone is UTC -5. Since the date/time is stored in server
time, Bob's previous order still shows that it was made on August 1 2007 at 9:00 AM. But since we are now in UTC -5,
it is as if Bob's order was made three hours earlier than it really was (since when it was 9:00 AM on August 1, 2007 in the
west coast it was really 12:00 noon on the east coast).
One way around this, you might contend, is to execute a SQL query that adds three hours to the order date for all records
in the table. Something like:
UPDATE Orders SET
OrderDate = DATEADD(hh, 3, OrderDate)
And such an approach would suffice... for this situation. But imagine that you moved to a web hosting company situated in
the US state of Arizona, where daylight savings is not observed. Eep. Now you would have to write a more complex UPDATE
statement that adjusted the hours based on whether the order date fell within daylight savings. Ick.
Another pitfall of server time is if you have multiple servers in multiple time zones. Now the values for each server's
date/time fields is relative to that server's geographical location. What UTC buys you is a single reference point.
Other Challenges of Date/Time Values
There are numerous other date/time-related challenges that I am going to ignore at this point. The thrust of this article
is to highlight UTC, how to use it in your data-driven applications for auto-generated timestamp-like date/time values,
and its advantages in that arena.
Other challenges of date/time values include displaying dates relative to one's time zone and daylight savings. This adds
some complexity as you must know the user's time zone offset and whether they are in a location that observes daylight savings
and whether the date/time value being displayed falls within the boundaries of daylight savings. But UTC is still a
better format to save dates than server time because it makes this complication a bit simpler. For example, if I know a
user's time zone offset is UTC -4 and I am storing date/time values in UTC, then I can simply subtract four hours to get the
correct non-daylight savings time. If I am using server time, then I have to determine the difference between my server time
and the users time zone and determine whether the server time is in daylight savings time and if the user's time zone is
observing daylight savings time. Double ick.
Getting the Current Time in UTC
Both the .NET Framework and Microsoft SQL Server provide functions to return the current time in UTC. In the .NET
Framework, use the DateTime.UtcNow property, like so:
' VB (use instead of DateTime.Now)
Dim currentTime As DateTime = DateTime.UtcNow
// C# (use instead of DateTime.Now)
DateTime currentTime = DateTime.UtcNow;
For SQL Server, use the getutcdate() function.
-- MS SQL (use instead of GETDATE())
DECLARE @currentTime datetime
SET @currentTime = GETUTCDATE()
Converting from UTC Time to Server Time
Given a time in UTC, the .NET Framework makes it easy to convert it to server time. Just use the DateTime class's
ToLocalTime() method,
like so:
' VB
Dim currentServerTime As DateTime = DateTime.UtcNow.ToLocalTime()
currentServerTime = CType(Eval("DateCreated"), DateTime).ToLocalTime()
A Complete Example
To further illustrate using UTC to store auto-entered date/time values, let's examine a sample application that has fields
to track when a record was created and last updated. This application, which is available at the end of this article,
has a single database table, Employees, with a very simple schema:
Employees
Column
Data Type
EmployeeID
int - Primary Key and AutoIncrement
FullName
varchar(50)
Salary
money
DateCreated
datetime
DateUpdated
datetime
In the download you'll find a single ASP.NET page, UTCFunctions.aspx, that contains a DetailsView control
for adding new employees and a GridView for listing and editing existing employees. Both are wired up to a SqlDataSource
on the page that contains the associated INSERT, UPDATE, and DELETE statements.
(For more information on working with data in ASP.NET 2.0, see my multi-part article series
Accessing and Updating Data in ASP.NET 2.0.)
As you can see from the screen shot above, the DetailsView does not allow the user to enter values for the DateCreated or DateUpdated fields.
Instead, it populates these with the current date/time in UTC. The INSERT statement used by the SqlDataSource
control follows:
For the DateUpdated value, the built-in SQL Server function getutcdate() is used. For the
DateCreated value, a parameter is specified (@DateCreated) and its value is specified programmatically
in the DetailsView control's ItemInserting event handler:
' VB
Protected Sub dvNewEmployee_ItemInserting(sender As Object, e As DetailsViewInsertEventArgs)
e.Values("DateCreated") = DateTime.UtcNow
End Sub
I used both techniques - getutctime() and DateTime.UtcNow - to illustrate both options. In practice
you'd probably want to choose one approach and use it for both parameters.
The GridView displays the DateUpdated and DateCreated values in UTC time. These are the raw values
from the database. The two fields' values are also converted to local time and displayed in columns in the grid. This conversion
is done using the ToLocalTime method. In particular, a TemplateField is used to show the local times and
the ToLocalTime method is used directly within the Label control's Text property:
Lastly, note that the GridView's editing interface does not allow the date fields to be modified. Whenever a record is edited
the DateUpdated field is updated to the current date/time (in UTC) via the UPDATE statement:
UPDATE [Employees] SET
[FullName] = @FullName,
[Salary] = @Salary, [DateUpdated] = getutcdate()
WHERE [EmployeeID] = @EmployeeID
Conclusion
UTC time provides a universal point of reference by which all time zones are offsetted from. Therefore, UTC is an ideal
choice for storing date/time formats in a web application when the web server and its users might reside in different time
zones. Things like daylight savings further complicate working with dates and times, but UTC does not observe any time zone,
simplifying things a tiny bit. In short, working with dates and times in a web application is one of those things that
is harder than it should be and has a lot of wrinkles that make the whole thing a bit of a mess. One step in the right
direction, however, is to make sure you are storing auto-entered timestamp-like date and time values in UTC rather than
in server time.
