Once again, we're not paid employees. There is no reason for such formality. Just like regular members, our activity goes up and down on a day-to-day basis. It's not like we plan it. We don't work in 'shifts' or have designated time off. We're online when we feel like being online and offline when we don't feel like being online, just like you all are. It's spontaneous, but most if not all of us do log into TBT on a daily basis regardless of whether we can only manage 5 minutes or 15 hours that day. If a mod is purposely taking time away (e.g. on vacation with limited interest access) we usually put it in our signatures or user titles. That's the only time it is really necessary for us to broadcast our activity levels to anyone.
For the record, despite complaints we're actually better now than ever in keeping on top of reports. Last summer I would log in each day to 25-50 reports as I was the only mod actively doing them before the August 2015 intake. That could take hours to go through depending on the complexity of each individual report. Now when I log in I never see more than 4-5, but most days it's zero and I only have to deal with new ones that come in while I'm actively online. So we're on top of the work as long as the members stay on top of reporting. If there are a couple of hours where there is no mod online it's really not as big a deal as people are making it out to be. There isn't really a way to guarantee 24/7 moderation and honestly it's not even necessary. It's an Animal Crossing forum not Buckingham Palace.
On that note: use reports. Post in Bell Tree HQ. We actually discourage people PM'ing us. It is much more likely that your problem will be dealt with faster by either of those two methods than if you're to PM one us individually.