Article
11/18/2011

DateAndTime.DateDiff Method (DateInterval, DateTime, DateTime, FirstDayOfWeek, FirstWeekOfYear)

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Returns a Long value specifying the number of time intervals between two Date values.

Namespace: Microsoft.VisualBasic
Assembly: Microsoft.VisualBasic (in Microsoft.VisualBasic.dll)

Syntax

'Declaration
Public Shared Function DateDiff ( _
    Interval As DateInterval, _
    Date1 As DateTime, _
    Date2 As DateTime, _
    DayOfWeek As FirstDayOfWeek, _
    WeekOfYear As FirstWeekOfYear _
) As Long

public static long DateDiff(
    DateInterval Interval,
    DateTime Date1,
    DateTime Date2,
    FirstDayOfWeek DayOfWeek,
    FirstWeekOfYear WeekOfYear
)

Parameters

Interval
Type: Microsoft.VisualBasic.DateInterval
Required. DateInterval enumeration value or String expression representing the time interval you want to use as the unit of difference between Date1 and Date2.

Date1
Type: System.DateTime
Required. Date. The first date/time value you want to use in the calculation.

Date2
Type: System.DateTime
Required. Date. The second date/time value you want to use in the calculation.

DayOfWeek
Type: Microsoft.VisualBasic.FirstDayOfWeek
Optional. A value chosen from the FirstDayOfWeek enumeration that specifies the first day of the week. If not specified, FirstDayOfWeek.Sunday is used.

WeekOfYear
Type: Microsoft.VisualBasic.FirstWeekOfYear
Optional. A value chosen from the FirstWeekOfYear enumeration that specifies the first week of the year. If not specified, FirstWeekOfYear.Jan1 is used.

Return Value

Type: System.Int64
Returns a Long value specifying the number of time intervals between two Date values.

Remarks

You can use the DateDiff function to determine how many specified time intervals exist between two date/time values. For example, you might use DateDiff to calculate the number of days between two dates, or the number of weeks between today and the end of the year.

Behavior

**Treatment of Parameters.**DateDiff subtracts the value of Date1 from the value of Date2 to produce the difference. Neither value is changed in the calling program.
Return Values. Because Date1 and Date2 are of the Date data type, they hold date and time values accurate to 100-nanosecond ticks on the system timer. However, DateDiff always returns the number of time intervals as a Long value.

If Date1 represents a later date and time than Date2, DateDiff returns a negative number.
Day Intervals. If Interval is set to DateInterval.DayOfYear, it is treated the same as DateInterval.Day, because DayOfYear is not a meaningful unit for a time interval.
Week Intervals. If Interval is set to DateInterval.WeekOfYear, the return value represents the number of weeks between the first day of the week containing Date1 and the first day of the week containing Date2. The following example shows how this produces different results from DateInterval.Weekday.
```
' The following statements set datTim1 to a Thursday
' and datTim2 to the following Tuesday.
Dim datTim1 As Date = #1/4/2001#
Dim datTim2 As Date = #1/9/2001#
' Assume Sunday is specified as first day of the week.
Dim wD As Long = DateDiff(DateInterval.Weekday, datTim1, datTim2)
Dim wY As Long = DateDiff(DateInterval.WeekOfYear, datTim1, datTim2)
```
In the preceding example, DateDiff returns 0 to wD because the difference between the two dates is less than seven days, but it returns 1 to wY because there is a seven-day difference between the first days of the respective calendar weeks.
Larger Intervals. If Interval is set to DateInterval.Year, the return value is calculated purely from the year parts of Date1 and Date2. Similarly, the return value for DateInterval.Month is calculated purely from the year and month parts of the arguments, and for DateInterval.Quarter from the quarters containing the two dates.

For example, when comparing December 31 to January 1 of the following year, DateDiff returns 1 for DateInterval.Year, DateInterval.Quarter, or DateInterval.Month, even though at most only one day has elapsed.
Other Intervals. Since every Date value is supported by a DateTime structure, its methods give you additional options in finding time intervals. For example, you can use the Subtract method in either of its overloaded forms: DateTime.Subtract subtracts a TimeSpan from a Date variable to return another Date value, and DateTime.Subtract subtracts a Date value to return a TimeSpan. You can time a process to find out how many milliseconds it takes, as the following example shows.
```
Dim startTime As Date = Now
' Run the process that is to be timed.
Dim runLength As Global.System.TimeSpan = Now.Subtract(startTime)
Dim millisecs As Integer = runLength.Milliseconds
```

The Interval argument can have one of the following settings.

Enumeration value	String value	Unit of time difference
DateInterval.Day	"d"	Day
DateInterval.DayOfYear	"y"	Day
DateInterval.Hour	"h"	Hour
DateInterval.Minute	"n"	Minute
DateInterval.Month	"m"	Month
DateInterval.Quarter	"q"	Quarter
DateInterval.Second	"s"	Second
DateInterval.Weekday	"w"	Week
DateInterval.WeekOfYear	"ww"	Calendar week
DateInterval.Year	"yyyy"	Year

The DayOfWeek argument can have one of the following settings.

Enumeration value	Value	Description
FirstDayOfWeek.System	0	First day of week specified in system settings
FirstDayOfWeek.Sunday	1	Sunday (default)
FirstDayOfWeek.Monday	2	Monday (complies with ISO standard 8601, section 3.17)
FirstDayOfWeek.Tuesday	3	Tuesday
FirstDayOfWeek.Wednesday	4	Wednesday
FirstDayOfWeek.Thursday	5	Thursday
FirstDayOfWeek.Friday	6	Friday
FirstDayOfWeek.Saturday	7	Saturday

The WeekOfYear argument can have one of the following settings.

Enumeration value	Value	Description
FirstWeekOfYear.System	0	First week of year specified in system settings
FirstWeekOfYear.Jan1	1	Week in which January 1 occurs (default)
FirstWeekOfYear.FirstFourDays	2	Week that has at least four days in the new year (complies with ISO standard 8601, section 3.17)
FirstWeekOfYear.FirstFullWeek	3	First full week in the new year

Examples

This example uses the DateDiff function to display the number of days between a given date and today.

Dim SecondDate = CDate(DateString)
Dim Msg = "Days from today: " & DateDiff(DateInterval.Day, Now, SecondDate)

Version Information

Silverlight

Supported in: 5, 4, 3

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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