In a worksheet I created several years ago, when you key enter a date
in the format of m/d, m/d/yy, or m/d/yyyy the result is a formula of
=m/d (and likewise =m/d/yy, =m/d/yyyy) a calculation of a decimal
fraction and the display of January 0, 1900, since the cell is
formatted as a date.

Opening a blank spreadsheet under the same preferences, exhibits
normal behavior--that is treating the entry as a date and formatting
it as a date.

Likewise in this particular worksheet, entering 2+3 (without the
usually required equal sign) results in =2+3 and displays 5.  Normal
behavior would display text 2+3.

I've looked at cell formatting, data validation, Tools>Options, etc.
without resolving this. The worksheet contains formatting, validation,
and VBA macros, but no attempt to change the.

I'd like to enter dates without the trouble of typing the formula
=Date(yyyy,mm,dd).

Actually it would be nice to know how turn this 'switch' on and off,
as I've always wondered why with all the 'features' in Excel it
required you to use the = sign for obvious calculations.