[This post has been significantly edited from its previous version: due to circumstances outside of my control; i.e. my own stupidity, I had the mechanics the wrong way round. I've made the appropriate changes and also updated things slightly.]
Time moves faster in Faerie and slower in Muspel. I've established that, but there ought really to be a way to mechanise it.
Let's begin with Faerie. It is almost always the case in literature involving travel to other, Faerie-esque worlds that time moves more quickly there: think of The Lion, the Witch and the Wardrobe, for instance. The characters disappear to another world where they have spent years, decades, a lifetime...and return to find that almost no time has passed in our world at all. (A recent update to this story is, of course, the Next Gen episode "Inner Light", which is one of the all-time great episodes of TV in any age or genre.)
So the mechanism for the movement of time in Faerie is simple: on returning to the "real world", the DM rolls a d100 and divides the amount of time spent in Faerie by the number on the dice. So 7 weeks in Faerie, with a result of 50 on the d100, means they have only been away from the real world for a little less than a day. To model the extremes to which these stories can go (the children in The Lion... are in Narnia for what seems to them like decades, but return only minutes later), the results explode downwards. If the result on the d100 is 1-10, the DM should re-roll, but as a d1000 and divide. If the result there is 1-100, the DM should re-roll as a d10000...and so forth.
That's simple enough. In Muspel, of course, things are the other way round. There, time moves much more slowly than it does in the "real world". Again, this can be discovered in myth: in an extreme example, the Japanese legend of Urashima Taro, the main character stays at the palace of the dragon god for 3 days and returns to his home town to find that 300 years have passed.
So the mechanism for the speed at which time moves in Muspel is almost the mirror image of Faerie. The rule is, when the PCs return from Muspel, the DM rolls a d20 for however long they have stayed there (approximately), and multiplies that amount of time by the number on the dice. For instance, if they've been there 6 hours, a result of 12 on the d20 means they've been there 72 hours. If they've been there three days, a result of 12 on the d20 means it was actually 36 days. However, a roll of 18 or above explodes upwards. The DM rolls a second d20, but this time the result indicates a higher time bracket (hours become days, days become weeks, etc.). So if the PCs were in the Muspel for a day, and the DM rolls an 18, he re-rolls; if the second roll is a 12, they were away for 12 weeks. If the second roll is a roll of 18 or above, it explodes upwards again, moving to a higher time bracket once more. This can quickly result in many years passing, in extreme examples.
Now, the question(s) then becomes: won't all this result in an insane level of complexity for the DM? Won't he have to have it worked out in advance what will happen years into the future?
The answer is "No", thanks to two mechanics, which I have yet to quite figure out yet in their entirety. The first of these uses the Series of Unfortunate Events method postulated by Pseudoephedrine on therpgsite years ago; for each month the PCs have been away, the DM rolls to see what has happened generally in New Troy since. (This takes work, yes: it is probably only to be done between sessions.) The second of these would be a table of Personal Events which is rolled for each PC, including deaths of family members, wives or husbands running off with somebody else, and so on - although this would perhaps be done on a yearly, rather than monthly, basis.
Faerie is much easier, of course: there, time moves more quickly. The only complication here is what people think when you go away and come back a few hours later looking incredibly old and with a whole load of new possessions and abilities.