You can create a new use field with exactly the same name as the tag in the file. Use the tag dump to see the name. This method will ensure you have the tag in MC always. The "mapping" can be achieved using auto-import rules.
Assuming it is RELEASETIME, you could then convert it to an MC internal date format with:
ConvertDate([RELEASETIME])
and assign it to a field, such as Date, by setting the Date tag with:
=ConvertDate([RELEASETIME])
If you don't want to create a custom field, and only want to populate some date field periodically, you can use Tag() to grab the physical tag for the occasional assignment:
=ConvertDate(Tag(RELEASETIME))
Don't use the leading = in an auto-import rule (it is only for setting the value of a tag in the UI).
The same concept applies with SUBTITLE. Just set an auto-import rule to set Description with the value of SUBTITLE.