The base unit of time is a "second". But it's a unit, so it doesn't have to be a "second" in the real world. However, the name is fixed, because there has to be something to conceptually anchor everything. As @weogarth pointed out, the more fanciful the calendar, the less anyone (especially the players but also the GM) is actually able to track anything based on that calendar.
We could potentially let you rename a "second", but I honestly don't see the benefits of doing so. And I'm NOT going to engage in a discussion about that subject right now - I have more urgent things to deal with. Once calendars percolate up the list, I'm sure one of the standard bearers for calendars will bring this up again.
The total span of time that the calendar mechanism can support varies, as you can carve up time into segments of your own choosing. The absolute limit of the entire time continuum is roughly 10^28 seconds. However, once you start carving things up into ages, epochs, years, seasons, months, days, hours, and minutes, you can lose a little of that. Regardless, you should have plenty of time span to comfortably model billions of years.
Oh, and the really nifty thing (IMHO) about the calendar design is that you can actually CHANGE THE CALENDAR without breaking all of the dates you've defined. So you can suddenly decide that you want to want to make the month of Granz a leap month after entering lots of dates based on the calendar. Or insert a new holiday season every 10th year after entering lots of dates. All of those dates will REMAIN VALID after you make the change. Any other calendar system I've ever seen would cause all existing dates to MOVE (read: break) after changing the underlying calendar. So the calendar mechanism lets you adapt it to the story you want to tell in interesting ways as your game evolves.
Alright, that's enough on calendars. I have lots to get back to here. But that should give all the calendar a sense of why calendars are the complex beast that we keep saying they are.
