How to modify a text file's "Date modified" property without affecting the content using c++?

I was doing a unit testing on one of my method. One criteria for the method to select the file into the result vector is the file is updated from last few hours. However, to ensure there is a valid result whenever I run the test, I need to keep at least one of the file "up to date".

Any idea about how to approach this except trying to copy the file to a temp file and write it back to the original file?