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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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Written by Angelo Arcoleo

Angelo Arcoleo, PMP,  brings over 30 years of experience in engineering, project management, planning and training.

He leads projects and teams to plan and execute critical projects utilizing his experience, quiet-leadership, passion and versatility to work with anyone.

He is a professionally trained civil engineer and has a bachelor of science degree from Rochester Institute of Technology. Angelo is a Master Scheduler for Harris Corp., formerly Exelis, in the Geospatial Systems Division. He holds an Orange Belt in Microsoft Office Project and is President of the Western New York Chapter of MPUG and a training consultant.

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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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