Looking for a tool to change a file's modified date which works from inside File Explorer context menu. I am aware of external tools like BulkFileChanger but I want to right click on a file and make the change.
This one is freeware, and will change what you need plus various attributes that users don't normally have access to:
You can use File Date Touch - Windows version of UNIX touch command
I know its probably too late to answer this, but apparently this http://stefanstools.sourceforge.net/SKTimeStamp.html does exactly what OP wanted:
SKTimeStamp is a very simple shell extension which adds a new tab to the Explorer properties dialog. On that new tab, you can change the file/folder date and time.
Available for x86 and x64 Windows systems.
You can change the create date from the command line. You must first change the system clock date, then open a command prompt window at the current working directory, and issue the command:
copy /b [filename] + ,,
This uses the concatenation directive with the binary (/b) switch, basically copying the file to itself and incidentally setting the create date of the "new" file to the current system clock.