Calc: date zero problems

[rogerl]

I realize that LibreOffice and Excel Win and Mac all use different date bases by default. I have a spreadsheet with a date base of 1/1/1904, and another spreadsheet with base 12/30/1899 (how did that happen?). That second spreadsheet has sheets that are many years old, and I want to preserve the data. All dates show properly, except that when I paste data from the first spreadsheet into the second, the dates are about four years too early.

What I would like to know is: is there a reliable procedure for converting both of those spreadsheets to a standard (say, 1/1/1899) date base? That way I can be reasonably assured that things will be consistent going forward.

[mariosv]

There is an option to select base date.

Menu/Tools/Options/LibreOffice calc/Calculate - Date

If I’m not wrong it is an option by file.

[rogerl]

It appears to be a global option. And in any case all it affects is new files (and the interpretation of existing ones). I guess I don’t see how I can use that to solve my problem.

[mariosv]

If your dates are e.g. in a column, and are values not formulas, you can use paste special to substract a value from them, to get the right value using the same base date than the file where you want to paste.