ACC: Storing, Calculating, and Comparing Date/Time Data

• Microsoft Access versions 1.0, 1.1, 2.0, 7.0, 97

SUMMARY

Moderate: Requires basic macro, coding, and interoperability skills.

This article explains how Microsoft Access stores the Date/Time data type and why you may receive unexpected results when you calculate or compare dates and times.

This article addresses the following topics:

• Storing Date/Time Data
• Formatting a Date/Time Field
• Calculating Time Data
• Comparing Date Data
• Comparing Time Data

MORE INFORMATION

Storing Date/Time Data

Microsoft Access stores the Date/Time data type as a double-precision, floating-point number (up to 15 decimal places). The integer portion of the double-precision number represents the date; the decimal portion represents the time.

Valid date values range from -647,434 (January 1, 100 A.D.) to 2,958,465 (December 31, 9999 A.D.). A date value of 0 represents December 30, 1899. Microsoft Access stores dates prior to December 30, 1899 as negative numbers.

Valid time values range from .0 (00:00:00) to .99999 (23:59:59). The numeric value represents a fraction of one day. You can convert the numeric value into hours, minutes, and seconds by multiplying the numeric value by 24.

The following table illustrates how Microsoft Access stores Date/Time values:

   Double         Date      Actual             Time      Actual
   Number         Portion   Date               Portion   Time
   ------------------------------------------------------------------
        1.0           1     December 31,1899   .0        12:00:00 A.M.
        2.5           2     January 1, 1900    .5        12:00:00 P.M.
    27468.96875   27468     March 15, 1975     .96875    11:15:00 P.M.
    33914.125     33914     November 6, 1992   .125       3:00:00 A.M.

   ? CDbl(#5/18/87 14:00:00#)
   31915.5833333333

   ? CDbl(#12/14/1849 17:32:00#)
   -18278.7305555556

   ? CVDate(1.375)
   12/31/1899 9:00:00 AM

   ? CVDate(-304398.575)
   8/1/1066 1:48:00 PM

Formatting a Date/Time Field

You can format a Date/Time value to display a date, a time, or both. When you use a date-only format, Microsoft Access stores a value of 0 for the time portion. When you use a time-only format, Microsoft Access stores a value of 0 for the date portion.

The following table illustrates how Microsoft Access stores Date/Time values and how you can display those values using different formats:

   Stored Value            Default Format    Custom Format
   (Double Number)         (General Date)    (mm/dd/yyyy hh:nn:ss AM/PM)
   ---------------------------------------------------------------------
   34834.0                 5/15/95           05/15/1995 12:00:00 A.M.
       0.546527777777778   1:07 PM           12/30/1899 01:07:00 P.M.
   34041.9375              3/13/93 10:30PM   03/13/1993 10:30:00 P.M.

Calculating Time Data

Because a time value is stored as a fraction of a 24-hour day, you may receive incorrect formatting results when you calculate time intervals greater than 24 hours. To work around this behavior, you can create a user-defined function to ensure that time intervals are formatted correctly.

This article assumes that you are familiar with Visual Basic for Applications and with creating Microsoft Access applications using the programming tools provided with Microsoft Access. For more information about Visual Basic for Applications, please refer to your version of the "Building Applications with Microsoft Access" manual.

NOTE: Visual Basic for Applications is called Access Basic in Microsoft Access versions 1.x and 2.0. For more information about Access Basic, please refer to the "Introduction to Programming" manual in Microsoft Access version 1.x or the "Building Applications" manual in Microsoft Access version 2.0

To calculate and format time intervals correctly, follow these steps:

1. Create a module and type the following line in the Declarations section:

         Option Explicit
   

2. Type the following procedure.

   NOTE: In the following sample code, an underscore (_) at the end of a line is used as a line-continuation character. Remove the underscore from the end of the line when re-creating this code in Access Basic.

         '------------------------------------------------------------------
         '  This function calculates the elapsed time between two values and
         '  formats the result in four different ways.
         '
         '  The function accepts interval arguments such as the following:
         '
         '     #5/12/95 6:00:00AM# - #5/11/95 10:00:00PM#
         '
         '     -or-
         '
         '     [End Time]-[Start Time]
         '------------------------------------------------------------------
   
         Function ElapsedTime (Interval)
           Dim x
           x = Int(CSng(Interval * 24 * 3600)) & " Seconds"
           Debug.Print x
           x = Int(CSng(Interval * 24 * 60)) & ":" & Format(Interval, "ss") _
              & " Minutes:Seconds"
           Debug.Print x
           x = Int(CSng(Interval * 24)) & ":" & Format(Interval, "nn:ss") _
              & " Hours:Minutes:Seconds"
           Debug.Print x
           x = Int(CSng(Interval)) & " days " & Format(Interval, "hh") _
              & " Hours " & Format(Interval, "nn") & " Minutes " & _
              Format(Interval, "ss") & " Seconds"
           Debug.Print x
   
         End Function
   
   

3. Type the following line in the Debug window (or the Immediate window), and then press ENTER:

         ? ElapsedTime(#6/1/93 8:23:00PM#-#6/1/93 8:12:12AM#)
   

   Note that the following values are displayed:

         43848 Seconds
         730:48 Minutes:Seconds
         12:10:48 Hours:Minutes:Seconds
         0 days 12 Hours 10 Minutes 48 Seconds
   

Comparing Date Data

Because dates and times are stored together as double-precision numbers, you may receive unexpected results when you compare Date/Time data. For example, if you type the following expression in the Debug window (or the Immediate window in earlier versions), you receive a false (0) result even if today's date is 3/31/95:

   ? Now()=DateValue("3/31/95")

To receive accurate results when you compare date values, use one of the functions below. To test each function, type it in the Debug window (or the Immediate window), substitute the current date for 3/31/95, and then press ENTER:

• To return an integer value, use the Date() function:

        ?Date()=DateValue("3/31/95")
  

• To remove the fractional portion of the Now() function, use the Int() function:

        ?Int(Now())=DateValue("3/31/95")
  

Comparing Time Data (in Microsoft Access 2.0)

When you compare time values, you may receive inconsistent results because a time value is stored as the fractional portion of a double-precision, floating-point number. For example, if you type the following expression in the Immediate window, you receive a false (0) result even though the two time values look the same:

NOTE: This behavior no longer occurs in Microsoft Access 7.0 or 97.

   var1 = #2:00:00 PM#
   var2 = DateAdd("n", 10, var1)
   ? var2 = #2:10:00 PM#

To receive accurate results when you compare time values, use one of the methods below. To test each method, type it in the Immediate window, and then press ENTER:

• Add an associated date to the time comparison:

        var1 = #1/1/90 2:00:00 PM#
        var2 = DateAdd("n", 10, var1)
        ? var2 = #1/1/90 2:10:00 PM#
  

• Convert the time values to String data types before you compare them:

        var1 = #2:00:00 PM#
        var2 = DateAdd("n", 10, var1)
        ? CStr(var2) = CStr(#2:10:00 PM#)
  

• Use the DateDiff() function to compare precise units such as seconds:

        var1 = #2:00:00 PM#
        var2 = DateAdd("n", 10, var1)
        ? DateDiff("s", var2, #2:10:00 PM#) = 0
  

REFERENCES

For more information about calculating time values, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q88657
   TITLE     : ACC: Functions for Calculating & Displaying Date/Time Values

Keywords          : kbprg kbusage TblDatyp kbfaq
Version           : 1.0 1.1 2.0 7.0 97
Platform          : WINDOWS
Hardware          : x86
Issue type        : kbhowto