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This works.

This doesn't. In lieu the file timestamp gets the actual date and time.

Although it doesn't seem to be documented, it appears the first parameter of FileSetTime is an expression. As a result, %Y%%M%%D% is a double-deref. Since no variable with the name 20000102 exists, it gets an empty value. Try using an expression as demonstrated:

Thanks, it works!

FileSetTime is the only command/function I know, that excepts a traditional and an expression variable.
This is slightly confusing. At least the intended behavior should be documented.

IIRC, only if(expression) and While(expression) don't support the traditional behaviour. I've tried briefly with WinMove; %var% and var are the same. I know this is also the case for Return. (%var%) and %var1%%var2% are always double-derefs, unlike %var%.

It seems only Return documents this.

Since this behavior has been in effect so long, it's probably best to leave it as it is. I've updated the documentation to read:

This parameter is an expression. Consequently, if multiple variables need to be concatenated to form a single timestamp, the dot operator should be used instead of percent signs. For example: FileSetTime, Year . Month . Day, C:\My File.txt

Thanks for reporting it.

The reason is that parameters that are pure numbers didn't seem to have any need for concatenation like %x%%y% (e.g. not many scripts would concatenate '5' and '6' to form the number 56). When expressions were added to the language, it seemed useful to support them natively in numeric parameters because it could be achieved without sacrificing backward compatibility. This is documented in the Expressions section:

Return's parameter was intended from the beginning to be a pure expression, and there was no need for backward compatibility. Unfortunately, it wasn't discovered until a while afterward that it behaved like it was backward compatible anyway. But rather than risk breaking existing scripts, it was left that way (maybe it was the wrong decision).

Also, since return's behavior was different than users might expect (since return can accept both strings and numbers), this was documented explicitly on its page.

Anyway, it seemed best not to repeat the mistake that was made with return for new syntax like while-loop that is intended to be a pure expression.

Thanks for the detailed explanation. I was aware of some of it, but mostly because I've been through the source code.
I guess time stamps are the only exception.
Although it demonstrates %var% and var are equivalent, it doesn't explain that this is an exception to expression syntax. For users that were not familiar with AutoHotkey before expressions were introduced, this might be confusing.

Thanks; I've added the following to the expression operator table for percent signs:

Thank you both for clarifying and for updating the documentation!

I thought I understood the previous posts here, but I am still puzzled. Why does this work:

and this does not:

variable oVar2 is in the "yyyyMMddHHmmss" format (actually it is the output of a FileGetTime of another script). I understand that because the 1st parameter is an expression, no %'s are to be used. But oVar2, w/o the %'s, is not recognized and the processed file(s) get today's stamp. Who can shed some light on this???.

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"Data is not information, information is not knowledge, knowledge is not understanding, understanding is not wisdom" but let's start to get the data...

This works:

Thank you, that helped me find what the problem was: a simple space ...

_________________
"Data is not information, information is not knowledge, knowledge is not understanding, understanding is not wisdom" but let's start to get the data...