For those interested, I've put together a 365.25 day calendar for use in the Planescape campaign setting. I originally created it eight or more years ago, I'm only now running a Planescape campaign so I've put some effort into reformating into a PDF format to be sent out to my players.
Although I know that other calendars exist for the setting, I was seeking to make a calendar that I felt was more in keeping with the abstract philosophic theme of the setting, one that might be in use across the planes rather than just just within the City of Doors. I also wanted it to reflect both a predictable (i.e. orderly) timekeeping system while including an ever-changing (i.e. chaotic) element.
Here's a rundown:
[sblock=Conceptual Calendar]The Cage is eternal, so much so that no one knows the date of its creation or founding. The Lady surely was present, but whether she and Sigil came into being simultaneously or she preceded the city is not know except by her. Those who stay in the city long enough realize that with the constant changes in Sigil it doesn’t matter too precisely how long ago something happened, but there are always those who take a keen interest in attempting to carve up eternity into manageable portions. Within the Cage, the modrons who operate the temple of Primus in the Lady’s Ward are recognized as the city’s official timekeepers.
THE CONCEPTUAL CALENDAR
A Guvner invention, the conceptual calendar is meant to represent a symbolic passage through all the planes of existence as well as their associated ideologies. Some bashers believe that the city’s mood often changes to reflect the philosophy associated with a particular moment, but most consider that mere screed.
	The clueless, along with some addle-coved berks, are often confused by the conceptual calendar. No surprise there; the Guvners have never been accused of making things easy for a body to understand. In truth most cutters don’t ever use the whole thing. Only detail obsessed graybeards delineate a moment as precisely as “the third hour before peak on chastity day, in the third rule of the state of Serenity, in the one hundredth and twenty-fifth year of Factol Hashkar’s reign”. Bar That. Most cutters simply use as little of the calendar as they need to be understood. Saying something like “last Lust night” or “in two rules and a day” is usually sufficient for most cagers’ needs.
Years
Also known as Cycles, each year is comprised of 365 days, including 17 months along with 8 or 9 civic holidays, known as cardinal days, which fall between every other state.
	Years are measured, then, from beginnings of factols’ rules, most often according to those of the Fraternity of Order. The current date is the 127th year of Factol Hashkar’s reign (often abbreviated as FHR). Some berks instead keep a tally of years starting with an event known as the Great Upheaval which occurred in Sigil some 630 years ago (equivalent to -503 FHR) in which the Lady of Pain is said to have intervened to dictate the number of factions which would be permitted to operate within the city.
Months
Also known as States, each of the 17 months is comprised of 3 weeks and represents one of the seventeen outer planes, all arranged in a sequence which mirrors the position of the plane within the Great Ring. The first state of the year, Balance, displaces the state that would typically follow the last state of the previous year. The cycle then continues on to the following state and so on until the cycle ends with the initially displaced state. In this way, the order of the months change from year to year in a 17-year “Great Cycle”.
	Since the order of months changes from year to year, they can be a confusing means of keeping track of yearly events such as anniversaries. In such cases most bashers simply count from the fixed cardinal days to keep track of specific events. For example, a cutter might remember his own birthday as being “one state and six days after Terra.”
 State			Associated plane
 Balance		The Outlands
 Order			Mechanus
 Concordance		Arcadia
 Exaltation		Mount Celestia
 Perseverance		Bytopia
 Serenity		Elysium
 Savagery		The Beastlands
 Passion		Arborea
 Valiance		Ysgard
 Chaos			Limbo
 Delirium		Pandemonium
 Depravity		Abyss
 Constraint		Carceri
 Despair		The Grey Waste
 Selfishness		Gehenna
 Dominance		Baator
 Conflict		Acheron
Cardinal days
These extra eight days in the calendar fall between states and represent the inner and transitive planes. Within Sigil they are observed as civic holidays of sorts, sometimes being celebrated with festivities which reflect the theme of their associated planes. Although the order of the states changes from year to year, the position of the cardinal days never waver. The first cardinal day, Astrum, occurs between the second and third state of the year. The following cardinal days then occur in order every two states thereafter. Once every four years a ninth cardinal day, called Prime, is added following  the state of Balance.
 Cardinal day		Associated plane
 Astrum			Astral
 Aer			Elemental Air
 Vita			Positive Energy
 Aqua			Elemental Water
 Aether			Ethereal
 Terra			Elemental Earth
 Mortis			Negative Energy
 Ignis			Elemental Fire
 Prime			Prime Material
Weeks
Also known as Rules, each week within a state is comprised of seven days and is named after one of the three planar axioms.
 Rules			Associated axiom
 1. Unity		Unity of Rings
 2. Center		Center of All
 3. Third		Rule of Threes
Days and Nights of the Week
Each period of light within the Cage is colloquially named after a virtue while each period of darkness is associated with a corresponding sin. 
 Day	  		Night
 1. Chastity	       Lust
 2. Temperance	   Gluttony
 3. Charity		Greed
 4. Diligence		Sloth
 5. Forgiveness		Wrath
 6. Kindness		Envy
 7. Humility		Pride
Hours
The bashers in Sigil base their timekeeping on hours relative to the peak hour of light. Peak is roughly equivalent to noon on a prime world; the six brightest hours in the City of Doors are the three hours before peak (BP) and the three hours after peak (AP). For the primes, this means 2 BP matches 1000 hours in military time, and 2 AP corresponds to 1400 hours. “Midnight” in Sigil is called antipeak. The six darkest hours come just before and after antipeak. Cutters should be aware that hours don’t have names or numbers, really (no Hour of the Weeping Crow, no terce or matins), just positions before and after peak and antipeak. This is clearest on the city’s clocks, which all have twenty-four increments and are shaded from black at the bottom (antipeak) to white at the top (peak).[/sblock]And here's a 
link to pretty formatted PDF download. Please let me know what you think. 
 
Edit: Fixed link