Firstly I'll apologise if this has been covered here, bu there is no search function (that I could find). If the answer to my question is just a URL then that would be great 
So, time in Sigil: I know that it is the 137th Year of the reign of Factol Hashkar when the PS box was published. And I know that each day in Sigil is 24 hours long. But, how are weeks, months and years measured? Is there any such thing as a week? A month? If so, how long are they? If not, is there an analagous construct?
I find myself hoping that a year isn't 365 days with 52 weeks and 12 months, as that would be pretty lame. I always thought that a 365 day year with 12 months of 30 days each broken down into 3 tendays (with 5 special days) as per Faerun was a reasonable way to keep timekeeping familiar yet different enough to be interesting. But that only works becuase Toril is said to similar to earth (ie same distance to sun, same orbital speed, etc. I do remember the claim it was much larger than Earth however). In the planes and Sigil all bets are off for that kind of reasoning.
Is there an official word on how to track time in Sigil? And if not, does anyone have a good idea? Once a time measure is established for Sigil, do the other planes follow this lead? Is a 24 hour day a multiversal constant (like big G maybe)? Once Hashkar dies or gets mazed or whatever, what happens then? Does Sigil just go back to the 1st Year of the reign of Factol Steve (or whatever)? If so it would seem like everyone in Sigil must be pretty good with their basic math in order to figure out how long ago certain events happened.
I guess it is possible that no ones cares, but given some of the Factions and their beliefs that would be highly unlikely I'd say (it wouldn't be surprising to find out the Xaositects had no concept (or care) of time, but the Guvners would most definitely have probably a very sophisticated time keeping mechanism). There would need to be some kind of common time keeping mechanism for much the same reason there is a common language - trade.
So, time in Sigil: I know that it is the 137th Year of the reign of Factol Hashkar when the PS box was published. And I know that each day in Sigil is 24 hours long. But, how are weeks, months and years measured? Is there any such thing as a week? A month? If so, how long are they? If not, is there an analagous construct?
They're known, but there's no formal definition for weeks and months. There's no moon to set a month by in many places, and where there is one (like on Karasuthra) it doesn't rise and set in normal fashion. The Planewalker's Handbook mentions that "a week" can mean anything from 5 to 10 days; it's a casual concept. Likewise, hours are handwaved, and minutes and seconds are off-the-cuff measurements -- timekeeping just isn't that good unless you're connected to the gears of Mechanus. We do know from The Great Modron March that a year is roughly one-seventeenth of a Grand Cycle, the length of time it takes Regulus to turn once (and the Great Modron March happens every 17 Cycles -- usually, that's something to set your watch by!).
It's clear that years do have some meaning on the planes. Bound fiends serve terms of specific numbers of years. Many, though not all, planes have seasons. Arcadia has exactly 12 months of exactly 27 days each, the seasons shifting every three months on the dot, for a year of 324 days.
Nope. On Ysgard, day and night are at the whim of the powers. On Pandemonium, there's no such thing. On the Beastlands on Krigala, it's always day.
That said, remember the planes are defined by mortal belief. Most Primes do seem to follow similar day and year length, and at least unconsciously mortals will conceptualize other worlds as doing the same. If most intelligent life evolves with diurnal rhythms, perhaps mortals who project to the Outer Planes subtly normalize their perceived time rate, so that inhabitants of one world seem slightly manic to observers while others seem slightly lethargic, but both think a "day" is the same length.
That said, there's no need your Prime Material crystal spheres need to have similar astronomical arrangements, or even physical constants! Maybe one human-inhabited crystal sphere has a world whose revolution is constantly dragging against its ether, spilling off enormous amounts of energy into the Ethereal Plane nearby and slowing down toward becoming tide-locked to its primary, so that even day lengths are changing. Maybe another world is a tiny planetoid with a very short day, so that the mortal race from there shows up in Sigil and is active around the clock, except that they sleep for about one hour out of every three. Other races think they're catnappers; they think everyone else has days 8 days long.
And they're also freaked out by how heavy the gravity is on most planes. Unless, again, they just normalize their perceptions. :^)
Yep! If something happened in the fifth year of the reign of Factol Lariset, you need to know how long Lariset ruled if you want to know exactly how many years ago it was. ("Now let me see... the contract was for ninety-nine and ninety-nine years, Haskar was installed 137 years ago, Lariset ruled for eighty (I made that up), so the fifth year of her rule was 137+75 years ago... contract's been up for twenty years now.")
I finally append that there are a couple of calendars for Sigil and the Outlands over on mimir.net: grab the "Outlands" and "Philosophical Calendar" downloads on http://www.mimir.net/torment/files/ .