Suppose you change a header foo.h, but your build system has dependency checking that will force the world to be rebuilt. A handy way to avoid that is with judicious use of the touch command, such as:
touch -t197301010000 foo.h
or (on Linux)
touch –reference another-file-that-is-early-enough-in-time foo.h
Both of these can be used to force the modification time for the file backwards.
This can be very useful way to save time.
It is also a great way to not build enough or break the build if misused!
I’d recommend this only for prototyping type work, to be followed up with a subsequent touch to restore the timestamps to normal, and then do a proper build.