Since measurement of time is usually based on celestial events, such as when the sun rises, when the phase of the Moon changes, or when seasons begin, I've arranged the natural cycles of the campaign so that they are easily comprehensible and will help stories rather than get in the way of them.
The Moon's cycle lasts exactly 28 days. The Sun's cycle is exactly 336 days. Why, that's exactly 12 times 28! How about that? Tom is making this up, so he decided to make it easy on himself and everybody.
There are 12 months, each named after one of the signs of the Zodiac, although different races and speakers of different languages will of course use their words for the constellations. Starting with the vernal equinox, the names of the months in the common tongue are: The Dragon, the Archer, the Goat, the Dolphin, the Lizard, the Forge, the Centaur, the Hunter, the Clock, the Hearth, the Serpent, and the Giant. The first day of spring is the first day of the month of the Dragon. Likewise, the summer solstice falls on 1 Dolphin, the autumnal equinox on 1 Centaur, and the winter solstice on 1 Hearth.
By the way, we are in the Southern Hemisphere here; in the Northern one the seasons are reversed, and the constellations and months probably have appropriate alternate names in the languages spoken there.
Each month has 28 days, and those 28 days are divided into 4 weeks of 7 days each. The days of the week are the same as ours. Simplicity is good. The week begins with Monday, for the simple reason that Monday is when the phase of the Moon changes, and Monday is the day of the Moon. The Moon is new on Monday the 1st of each month, hitting first quarter on Monday the 8th, full on Monday the 15th, and last quarter on Monday the 22nd. Having this all laid out allows for easy determination of such things as ocean tides and the cycles of female human characters (which can be linked with the Moon if the players of those characters so decide).
In the common speech, dates are given as something like this: "Today is Wednesday the 17th, in the month of the Archer." In writing it might be abbreviated as "Wed 17 Archer." Other languages may do this differently. Not that we will actually be speaking other languages across the table in any likelihood, but elven characters could speak to each other in the elven tongue, even though the players would be speaking English.
Of course, different cultures will celebrate differently, but the solstices are usually marked by big parties or ceremonies of some kind. There are also the equinoxes and the full moons.
In Mirrhenia, and consequently in surrounding lands, the full moon at midsummer (15 Lizard) is a big holiday, as is Yule (1 and 2 Hearth).
There has been no time travel in any storyline so far. It's complicated.