尝试在更新单个记录中的特定字段时捕获日期和时间

I had to add the fields Comments and Comments DateTime to a table. A user would leave comments, and when this was complete, the Comments DataTime is supposed to capture the date and time of the update of the Comments field. Many examples I've seen track when the entire record updates a datetime field, however I'm looking to capture the datetime only when the specific Comments field is update for that record. How do I go about doing this? Any advice is greatly appreciated.I had to add the fields Comments and Comments D