Generally, fields that MC uses automatically are set to either read-only or clear-only to protect the data integrity of the Library. Matt explains some of his reasoning
in this thread.
If you want to edit it manually, you can, though, due to the "bug" they were discussing in the above-referenced thread.
Technical Description: Because the expression engine inlines field values for performance purposes, if you reference a field in another calculated field (or expression column), you can use that to set otherwise read-only values.
So, for this example, you can make an "editable" version of the [Date Imported] field like so:
1. Go to
Options > Library & Folders and choose Manage Library Fields. Add a new field and call it "Date Imported Editable" (or something similar).
2. If desired, you can change the Display (singular and plural) settings to make it match the regular [Date Imported] field, or something that looks nice when used in a column.
3. Change the type of this new field to Calculated Data.
4. In the expression box, use this:
=[Date Imported]5. Use this new Field in your Columns or Tag Action window or from wherever you'd like to be able to set this manually. Be aware, this will let you put ANY data you want into the [Date Imported] field, which means you could put nonsensical stuff in there. You're going to have to learn how MC parses date entries.