Setting a file to the current date is called touching it. This is handy to, say, recompile all your C++ source files, regardless of their dates. Or, to use old object files without recompiling the source files.
The touch utility changes the modification date of files to the current date. touch commands have the form:
touch file1 file2 ...
Use touch with smake to maintain the dates of files and to modify the operation of smake. For example, to recompile all the C++ source files that comprise a program (not just the ones last modified), use the following commands:
touch *.cpp smake
Also, if you edit one or more files, but change only comments or other items that do not affect the resultant object file, then touch does not remake the object files.
touch *.obj smake