Hi,
Lets not talk about TMG when you are doing a direct update to the table using a report.
TMG is when you update the table through the TMG itself, not a report.
So to update the table through report, yes you need to fill current date and time in the table fields for date and time along with other fields and send update to the table.
However, your requirement could have been achieved, if the direct update through a report somehow would have triggered the TMG (where we could have written code to update date and time), but that's not the case, and the SAP design is not such. It doesn't trigger the TMG program of the table when sending the direct update to table; it's a direct interaction of your report with the table in database.
Thanks..