I keep track of attendance in Moodle's grade book by creating grade items for each date. I like to give students 3 points if they attend, 1 point if they do not attend but let me know, and 0 points if they do not attend and do not notify me.
If I was faced with your scenario, I would simply add another grade item for 3/15, put it into the right position in the grade book, and enter the grades.
I also like to put all of these attendance items into their own grade book category called "Attendance and Participation."
(See attachment as an example.)
