".../files/scripts/person/person.gd" is the person class, which has the "metrics" variable to track how many times various things have happened to the slave, but there is nothing about dates in there. The person class uses "lastinteractionday" to track when and how many times the player has used interactions on a slave, but it doesn't differentiate between meet/date and sex.
"metrics" is the best place to add a variable to count how many dates a slave has had. Note that changing the contents of the variable in the file will not automatically update the contents of the variable for save files created prior to the changes to the file. You will need to add code to add the necessary entries to the variable if you want it to be compatible with existing saves.