Determining Differences Between Dates

Another small difference between VBA and Worksheet function:
VBA:
=DateSerial(200, 2, 28) will evaluate to a serial number for Feb. 28, 0200

Worksheet function:
=Date(200,2,28) will evaluate to Feb. 28, 2100. Weird!

ON that weird note, both functions accept parameters that are out of range like 35 or 61 for day, and 13 or 24 for month. For day and month the negative numbers are accepted too!
e.g.,
=Date(2024,-6,1) --> June 1, 2023
=Date(2024,16,-15) --> May 16, 2025
This can be used creatively, for instance to get the date that is 44 days from the Mar. 1, 2024 =DATE(2024,3,45).

When using date calculations in Excel you have to be aware of some idiosyncrasies, and differences between dates in the worksheet and in VBA.
Let’s first clarify the statement from the tip: “The serial number represents the number of days elapsed since a starting "base date," specifically since 1 January 100. (Yes, that's the year 100.)”

While it is true that the earliest VBA date is January 1, 0100, the serial number assigned to this date is not zero nor one but -657434. The day assigned the serial number 1 in VBA is actually Dec. 31, 1899. Hence, Jan. 1, 1900 is assigned serial number 2. This is significant, because that date (Jan. 1, 1900 ) in worksheet is assigned serial number 1. Why this discrepancy? Because in the worksheet, the year 1900 is treated as a leap year, **which it is not**.

This one-day discrepancy carries through from Jan. 1 to Feb 28, 1900. The worksheet will even accept 1900-02-29 as valid date, but there was not such a day in Gregorian calendar. Starting with March 1, 1900 both worksheet and VBA match the date serial numbers, so for most common uses this is not an issue. The conclusion of this is that you need to keep it in mind, if any of the dates you use in your calculations are in this two-month period.

VBA handles the leap-year calculation correctly, assigning negative numbers to pre-1900 dates. However, there is no **simple way**, i.e., no format, that would display a pre 1900 date in a worksheet. Negative serial numbers formatted as date show as string of “#” filling the cell. On the other hand, you can use various tricks to handle those dates, some of them described in another tip:
https://excelribbon.tips.net/T011082_Using_Early_Dates
There is a link there, that is now outdated, to another web resource; use the following instead:
https://exceluser.com/how-to-work-with-dates-before-1900-in-excel/

On top of that, there is an issue of dates in Gregorian vs. Julian calendars. The switch was first implemented in four countries and the first date in this Gregorian calendar was Oct. 15, 1582. France was next on Dec, 20,1582. Other countries followed at different times until 1927! UK, USA and Canada started on Sep. 14, 1752. In all countries that switched from Julian to Gregorian calendar there was a missing period of 10 (Italy) to 13 days (Turkey) that were removed from the calendar. Excel VBA does not account for that (it really cannot, as it is different depending on the country). It just applies Gregorian dates backwards all the way to January 1, 100. The short of it is that if you want to count number of days between two dates you have to manually account for this discrepancy. How to do this is a whole different story. Probably the best approach would be to first convert any Julian date to equivalent Gregorian one, and only then do the math.