DT_SUBTRACT Function

Decrements the values in a date/time variable by a specified amount.

Usage

NOTE: Only one keyword can be specified at a time. You cannot, for example, specify both years and months in a single DT_SUBTRACT call. But if you need to subtract, for example, one day and one hour, you can simply subtract 25 hours.

Discussion

The DT_SUBTRACT function returns a date/time variable containing one or more dates/times that have been offset by the specified amount.

The keywords specify how the dates and/or times are decremented (subtracted from). If no keyword is specified, the default decrement is one day.

Only positive whole numbers (including zero) can be used with the keywords to specify a decrement. Therefore, the smallest unit that can be subtracted from dt_var is one second.

Example 1

dtvar = VAR_TO_DT(1992, 03, 17, 09, 30, 54)

; Create a date/time variable containing a date/time.

dtvar1= DT_SUBTRACT(dtvar, Year=4)

; Create a new date/time variable by subtracting 4 years from dtvar.

PRINT, dtvar1

{ 1988 3 17 9 30 54.0000 86017.396 0}

; Display the new date/time variable.

Example 2

This example shows how to add one day to a date/time variable containing two date/time values.

dtarray = STR_TO_DT(['17-03-92',  $
    '8-04-93'], Date_Fmt=2)

; Convert two date strings to a date/time variable.

DT_PRINT, dtarray

03/17/1992
04/18/1993

; The date/time variable dtarray contains two dates.

dtarray1 = DT_SUBTRACT(dtarray, /Day)

; Create a new date/time variable dtarray1 that contains two 
; dates with one day subtracted from each date.

DT_PRINT, dtarray1

03/16/1992
04/17/1993

Example 3

This example shows the effect of using the Compress keyword with DT_SUBTRACT. Assume that you have defined Christmas (December 25, 1992) to be a holiday with the procedure CREATE_HOLIDAYS.

x = VAR_TO_DT(1992, 12, 26)

; Begin with a date variable containing December 26, 1992.

y = DT_SUBTRACT(x, /Day, /Compress)

; Subtract one day from the variable.

DT_PRINT, y

12/24/1992

; The result is December 24. Normally, the result would be 
; 12/25/92, but because December 25 is defined as a holiday,
; the Compress keyword causes the 25th to be skipped.

DT_SUBTRACT Function

Usage

Input Parameters

Returned Value

Keywords

Discussion

Example 1

Example 2

Example 3

See Also