You can initialize a Date variable to a literal value by surrounding it in number signs as shown here:

Dim dueDate As Date = #10/31/2011 1:23:00 PM#

You can also initialize a Date by using the New keyword as in the following code:

Dim dueDate As New Date(2011, 10, 31)

The preceding code uses a year, month, and day to initialize its Date variable, but the Date type lets you use many different kinds of values. The three most useful combinations of arguments specify the following (all as integers):

You can also add a kind parameter to the end of the second and third of these combinations to indicate whether the value represents local time or UTC time. (Local and UTC times are explained in the next section.) For example, the following code creates a Date representing 12 noon on March 15, 2010 in the local time zone:

Dim idesOfMarch As New Date(2010, 3, 15, 12, 0, 0, DateTimeKind.Local)

Windows has several different notions of dates and times. Two of the most important of these are local time and Coordinated Universal Time (UTC).

For most everyday tasks, local time is fine. If you need to compare data on computers running in different time zones, however, UTC time can make coordination easier. For example, if you want to know whether a customer in New York created an order before another customer created an order in San Salvador, UTC enables you to compare the times without worrying about the customers' time zones.

A Date object's Kind property indicates whether the object represents local time, UTC time, or an unspecified time. When you create a Date, you can indicate whether you are creating a local time or UTC time. If you do not specify the kind of time, Visual Basic assumes you are making an unspecified time.

After you create a Date, the type provides a couple of methods for converting it between local and UTC values. The ToLocalTime method converts a Date object to local time. Conversely, the ToUniversalTime method converts a time to UTC time.

The Date type provides many useful properties and methods for manipulating dates and times. Table 15-1 summarizes some of Date's most useful methods. It indicates which methods are shared, meaning you invoke them using the type name, rather than a variable name, as in Date.IsLeapYear(2012).

Table 15-2 summarizes the Date's most useful properties.

A Date represents a point in time (July 20, 1969 at 20:17:40). A TimeSpan represents an elapsed period of time (1 day, 17 hours, 27 minutes, and 12 seconds).

One of the more useful ways to make a TimeSpan is to subtract one Date from another to find the amount of time between them. For example, the following code calculates the time that elapsed between the first and last manned moon landings:

Dim firstLanding As New Date(1969, 7, 20, 20, 17, 40)
Dim lastLanding As New Date(1972, 12, 11, 19, 54, 57)
Dim elapsed As TimeSpan = lastLanding - firstLanding
MessageBox.Show(elapsed.ToString())

The code creates Date values to represent the times of the two landings. It then subtracts the first date from the second to get the elapsed time and uses the resulting TimeSpan's ToString method to display the duration. The following text shows the code's output in the format days.hours:minutes:seconds.

1239.23:37:17

Table 15-3 summarizes the TimeSpan's most useful properties and methods.

Note that you can use the + and – operators to add and subtract TimeSpans, getting a new TimeSpan as a result. This works in a fairly obvious way. For example, a 90-minute TimeSpan minus a 30-minute TimeSpan results in a 60-minute TimeSpan.

In this Try It, you use Date and TimeSpan variables to build the stopwatch application shown in Figure 15-1. When the user clicks the Start button, the program starts its counter. When the user clicks the Stop button, the program stops the counter.

Figure 15.1. FIGURE 15-1

Normally the TimeSpan's ToString method displays a value in the format d.hh:mm:ss.fffffff. In this example, you use String.Format to display the elapsed time in the format hh:mm:ss.ff.