What Is the File Modification Date?

Date modified, also called the modification date or modification time, is the date and time a file was last changed. In a list of metadata, the field is called File Modification Date/Time. The ExifTool program names the tag FileModifyDate.

This date is not stored inside the file. The file system on your disk keeps it, next to the file name and the file size. ExifTool calls such values pseudo tags, because they are not real metadata in the file.

Which files carry it

Every file has a modification date, whatever its format. Photos, songs, videos, and documents all get one from the file system.

Windows File Explorer shows it as Date modified, and Windows programs call it the last write time. Systems that follow the POSIX standard call it the last data modification timestamp.

What the value looks like

ExifTool shows the year, month, and day separated by colons, then the time and the time zone offset. The ExifTool documentation shows this example:

• 2009:10:05 20:40:36-04:00 means 5 October 2009, at 8:40:36 in the evening
• -04:00 at the end means 4 hours behind UTC, and a plus sign means ahead of UTC

On Windows, the NTFS file system stores the date in UTC. The time you see then depends on the time zone of your computer. In an example from Microsoft, a file saved at 15:00 Pacific time shows 18:00 on a computer set to Eastern time.

The older FAT file system stores local time instead, so the same file shows 15:00 in both places. FAT also stores the modification time in steps of 2 seconds.

What changes the date

The date moves to the current time when a program writes new data into the file, for example when you save a document. Writing new metadata into the file, such as a photo tag, also changes the date. ExifTool sets this date to the current time when it writes a tag, unless you use its -P option.

Reading a file does not change the modification date. Reading changes a different value, the last access date. Copy programs differ: some keep the original modification date, and others give the copy the current time.

You can also set the date yourself. The POSIX touch command sets it to the current time or to a time you choose. ExifTool can copy the date a photo was taken into the modification date.

Modification dates inside the file

Many formats also store a modification date inside the file. Only a program that edits the file can update it, so it can differ from the file system date. Some examples:

• In JPG photos, the EXIF tag ModifyDate, ID 0x0132, holds the date and time the file was changed. A separate tag, OffsetTime, can hold its time zone.
• In PDF files, the document information holds a ModDate entry.
• In MP4 and MOV videos, the movie header holds a ModifyDate, which should be stored in UTC.

Why it matters

Date modified tells you when a file was last saved, not when it was made. For the moment a photo was taken, see Date/Time Original. For the moment a file was first made, see Create Date.

The value is only as correct as the clock of the computer that saved the file. Programs can also set it to any time, so it does not prove when a file was really changed.

Backup programs can use this date to find files that changed. The Windows Robocopy command, for example, can skip files by the date they were last modified.