The last modified date for the subscription status field is being logged as of October 1, 2007.
This field can be exported, imported and used in filters.
The filed in export/import files is called “subscribelastmod” and it uses the following format:
For example: 2007-10-01 15:20:00 (October 1, 2007, 3:20pm)
Please note that data is not available for contacts where the last modified date of their subscription status is before October 1, 2007.