As they said, it only accumulated while in a sanctuary. It’s not a timer that will count down from 48 hours (not to mention gaining any exp would have to readjust the timer). Airships, subs, beast tribes, company crafts (the grade 3 buff wheels) all get a timer because they all have set times.

A timer for rested EXP would need to take into account being in a sanctuary, being able to pause while outside of a sanctuary, and be able to recalculate on the fly should you gain exp before it’s been capped. If they were able to successfully do this then imo they should look into adding a gardening timer into the timers menu.