Detect when the player is mining/ herbing?

[Malsomnus]

My add-on absolutely needs to also remind players if they haven't been to their garrison's mine/ herb garden yet today, and I figure that the best I can actually do is detect when the player has done some mining/ herbing (yes, it is a verb) while inside the garrison zone.
The best way I came up with is checking the combat log to see when the player finishes casting the mine/ herb spell and then disabling the reminder for this toon on this date, but I'm sure that as usual there's probably a better way that hasn't occurred to me, so... does anyone know how I could improve the above idea?