About Time

In Access Answers, Doug Steele addresses commonly asked questions from Access developers. This month, he looks at  Validate input such as ZIP codes and postal codes  as well as problems dealing with time values.

I’m trying to add together time values, but Access won’t let me exceed 24 hours.

The Date/Time data type in Access really isn’t intended to be used for time durations. Rather, it’s intended to be used for timestamps: specific date/times. When you save a time without a date, Access actually stores the time as if that time occurred on 30 Dec, 1899. This is because of how date/time fields are stored. A date/time is stored as an IEEE 64-bit (8-byte) floating-point number. The integer part of the number represents the date as the number of days relative to 30 Dec, 1899 (for instance, 04 Apr, 1897 is -1000, 04 Jan, 1899 is +5, and so on). The decimal part of the number represents the time as a fraction of a day (in other words, 3:00 AM is .125, 8:00 AM is .3333333, Noon is .5, and so on).

So, if you add 3:00:00 + 8:00:00 + 6:00:00 + 8:00:00, you’re adding together .125 + .333333 + .25 + .3333333, which is 1.041667. Rather than getting 25:00:00, as you’d expect, you get 31 Dec, 1899 01:00:00, as this example from the Debug window shows:

So what can you do? There are two possibilities. The preferred way—and the one I’ll concentrate on—is to use a different data type to store your times. For example, if you need resolution to the second, use a long integer, and store your times as the number of seconds (three hours becomes 10,800, eight hours is 28,800, and so on). If you add these together, your sum is going to be 90,000 seconds. All you need is a function to return that number in normal time format. Here’s that function:

You’ll probably also want to have a function that will convert from time to time in seconds:

Just to prove what I said previously about the times being in seconds:

Now you can use the two functions in combination to calculate durations:

In your tables you should be storing durations in seconds in a Long Integer field, rather than in a Date/Time field. A SQL statement that would find total time would be:

If, despite what I’ve written here, you’re determined to stay with the date/time type, the alternative is to write a function to reformat the value you got from your sum:

You can use that function in conjunction with normal times:

So now, if your durations are in the table as Date/Time fields, a SQL statement to find total time would be:

Okay, that’s about all I have time for this month <groan>.


 Your download file is called   Steele_AboutTime.accdb 

About Doug Steele

Doug Steele has worked with databases-both mainframe and PC-for many years with a major international oil company. In a previous life, he taught introductory computer programming at the University of Waterloo. Microsoft has recognized him as an Access MVP for his contributions to the user community over the years.

Doug is the author of Access Solutions with Arvin Meyer.

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