[CMake] Cross-platform way to get timestamp in cmake script?

[Miguel A. Figueroa-Villanueva]

Hello,

Does anyone here have a cross-platform way to get a timestamp in a
CMake script. I want to put a signature in a file I am writing to with
the file(WRITE ...) cmake command such as: "Last modified on May 03,
2009."

Thanks,
--Miguel