How does your business process track the next due maintenance action?

I wanted to start a poll, but I didn't find that option. So, here's the question:

How does your business process track the next due maintenance action?

1) Using a calendar counter. Such as, every 6 months this maintenance requirement is due to unit xyz. Then unit xyz is scheduled to be in for maintenance every 6 months.

2) Using a flight hours counters. Such as, this engine has 4,000 hrs left until maintenance requirement is due. In this case planning, perhaps outside of cMRO, is required to ensure that engine comes back in after 4,000 hours of use.

3) Event driven, like after x number of landings, due maintenance requirement abc.