I want to use the modified date of the file to populate a id3 tag

I have a digital recorder and would like to preserve the file modified date as this indicates when the recording was made. The recorder itself (Roland Edirol) doesn't add id3 tags to the files. I would like to use mp3tag to take the modified date and put it in an id3 tag field, maybe comments. I have tried to use a custom actions and see the option to use %_file_mod_datetime% but I am unclear if I can grab it (before it changes because of the edit to the tag) and populate comments with it. If so, can someone walk me through how to do so please?

You can use an action or a group of actions like the following example:

Begin Actionsgroup Save FILE_MOD_DATETIME to tag

Action #1
Actiontype 5: Format tag field
Formatstring: %_file_mod_datetime_raw%

Action #2
Actiontype 5: Format tag field
Formatstring: %_file_mod_datetime%

End Actionsgroup Save FILE_MOD_DATETIME to tag (2 Actions)

You have only one opportunity!
You can resp. should run the action only once against the file, afterwards the file get the current datetime at the moment when the file has been changed by writing the tagfield into the file.

You may use the COMMENT tagfield instead or any other name of your choice to create some new tagfield.