The people of Vralkast have no sun to track the days, they instead track time via the tides. There are 26 tides to a Tide Cycle, and 13 Tide Cycles to an Olmtide. This means that the Fomorian year is 2 days longer than the Galifaran year. For ease of conception, a Tide is still 24-hours. High Tide occurs once every 24 hours, at what we would consider Midday. Low Tide occurs at Midnight.
To convert between the two calendars, do the following:
A Galifaran year is 336 days, split between 12 months of 28 days
A Fomorian year is 338 days, split between 13 tide-cycles of 26 tides
The largest measure of time they share is a day, so for example, the 5th of Sypheros is the 285th day of the year. For ease, we will assume the Fomorian calendar also started on the same date, though this wouldn't be the case.
5/10/999 YK is 335949 days ((336 * 999) + 285)
Now, to convert this to the Fomorian calendar you do that in reverse
335949/338 gives you 993.931952663
So it's the 993rd year. It's 93% of the way through the year, so the day is 0.931952663 * 338, which is the 315th day. Divide that by 26 (the number of tides in a tide-cycle) to get 12.1153846154. A tide-cycle is 26 tides, so a single tide is 0.03846153846 of a tide-cycle. Divide 0.1153846154 by 0.03846153846 to get 3. So, 5/10/999 YK is 3/12/993 on the Fomorian calendar.