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The 0.38 day Mystery Solved!

Mystery_January_1934 (2)

Ever wonder where that pesky 0.38 days comes from that you find when comparing dates that look to be the same?

In a recent inquiry, a reader wanted to know, “Why does a ‘one working day’ task show up as 0.38 calendar days?”

In his schedule he was using this formula in a Text1 field:


I tried a few formulas and different fields to see if this behavior was field-related, with no satisfactory answer. So I reached out to my guru, Barbecana Chief Operating Officer, John Owen! Well, he figured out where the 0.38 comes from.

Microsoft Project uses a numeric representation of Start and Finish dates, and it’s actually a decimal fraction of a day.

Given a start date of:

10/28/16 8:00 AM

and a finish date of:

10/28/16 5:00 PM

Microsoft Project converts that to 42671.33 for Start and 42671.71 for Finish.

The .33 at the end of start number is 0.33 of 24 hours, which equals 08:00.

The .71 at the end of finish number is 0.71 of 24 hours, which equals 17:00.

0.71 – 0.33 = 0.38

Mystery solved!

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  1. Ha! I always wondered why that happens. Thx to John Owen for figuring this out… but I woulda figured that he knew, as that full monte add-in is insane with calculations!

  2. Actually the calculation work out to be 0.375 in fact but it gets rounded up ! A simple way to think of this problem is that out of a 24 hour calendar day 8.00 am – 5:00 pm represents 9h. So 9 divided b 24 is 0.375 as a decimal fraction. And so if you know the number of hours in the day you can simply divide by 24 to get the fraction. 🙂


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