Enigma?

[BobH]

No, not the famous code-breaking machine, but something I ran across.
In the attached file you will see 12 lines each with a begin and end date followed by the calculated number of days between those dates. The final row shows the sum of the number of days calculated and the calculated number of days from the first date in the first row to the last date in the last row.

The enigma is why numbers aren't the same. I've looked at this until I can no longer 'see' the problem.

enigma.xlsx

[HansV]

Let's take row 2 as example. The start date in B2 is March 21 and the end date in C2 is April 20.
The formula in D2 =C2-B2 returns 30: it is 30 days from March 21 to April 20. But the period starting at March 21 and ending at April 20 has 31 days - it includes both the start date and the end date. To calculate the length of the period, you should use =C2=B2+1.
If you use that formula and fill down to D13, the sum in D14 changes to 365 - the number of days in a (non-leap) year.
The corresponding formula in E14 would be =C13-B2+1; this also returns 365.

[BobH]

Thanks, Hans!

I had developed a mental block from looking at it too long.