Folks may use the term "Module Timestamp" to mean both file timestamp and image header timestamp. Although they're usually very close, they're different and won't be identical. Here's a compare/contrast:
The image timestamp is what you see under a debugger. For example, windbg's 'lmv' command displays the timestamp as both the raw 32-bit value and a conversion to something useful:
Image name: notepad.exe
Timestamp: Tue Aug 03 23:05:55 2004 (41107CC3)
The filetimestamps can be viewed from Explorer. Just right click on the file and bring up properties. For comparison, the timestamps from the same file via the file system: Created: Monday, August 09, 2004, 11:11:33 AM Modified: Wednesday, August 04, 2004, 4:00:00 AM Accessed: Today, January 18, 2007, 7:22:56 PM
The image timestamp (and other related data) is also what's captured in a dump file (see MINIDUMP_MODULE). So when a debugger wants to correlate modules in a minidump to real modules on disk, it can use the timestamp and checksums in the image header. This is similar to how PDB matching works.
While I'm on topic, Raymond has an excellent overview of different time formats.