I will make reference to a number of different methods of measuring a year, and although I won't explain their rules, they are out there to look up if you want. The measurements I will refer to are:

Tropical / Solar Year: Approximately 365.24219907 days (365 Days, 5 hours, 48 minutes, 46 seconds).

Simple year: Exactly 365 days. (Not used by any culture that I know of.)

Julian Year: Average of 365.25 days with a cycle of 4 years (365 days 6 hours).

Gregorian Year: Average of 365.2425 days with a cycle of 400 years (365 days 5 hours 49 minutes, 12 seconds).

New Era (Farede) Year: A year according to the calendar in general use in the Wheel of Time.

366 days in years divisible by 4 (Feast of Thanksgiving added). (996, 1000, 1004, etc.)

366 days in designated year within 10-year cycle, presumably the years divisible by 10, but unconfirmed within the books (All Soul's Day added) (400, 990, 1000, etc.)

367 days in years where above two conditions correspond (980, 1000, 1020, etc.)

365 days in all other years (997, 998, 999, etc.)

Average: 365.35 days per year (31566240 seconds).

Since the tropical year does not contain an exact number of calendar days, we cannot have the same number of days from year to year without the calendar year diverging from the tropical year. For instance, if our calendar year had 365 days in it every year (instead of 366 every 4 years), it would diverge from the tropical year by 5.8 hours every year. Not sound like much? If Pope Gregory XIII had championed the simple calendar in 1582 instead of the Gregorian Calendar, the first day of Winter would now be April 2.

So how much does each one diverge each year?

Simple Year: -.24219907 days (- less than 6 hours)

Gregorian Year: .00030093 days (26 sec)

Julian Year: .00780093 days (a little more than 10 minutes)

Farede Year: .10780093 days (about 2 1/2 hours)

And in a 500-year span?

Simple Year: -121.09954 days

Gregorian Year: 0.15047 days

Julian Year: 3.90047 days

Farede Year: 53.90047 days

And in a 1000-year span?

Simple Year: -242.19907 days

Gregorian Year: 0.30093 days

Julian Year: 7.80093 days

Farede Year: 107.80093 days

In the case of the Simple Year, Spring would come later every calendar year. In the case of the other three, Spring would come earlier every calendar year. And in the case of the Farede Calendar, Spring would have come earlier every calendar year until, in 1000 NE, it was arriving at approximately our equivalent of December 21.

Possible explanations

#1 - Jordan's oversight: Robert Jordan may not have carefully considered the long-term implications of the calendar used in the Wheel of Time.

#2 - Different Solar year: It is known that planets' orbits are capable of drifting over a period of millions of years. If it were to drift, the length of the tropical year would drift accordingly, and a 365.35 day year could be the most accurate.

#3 - Primitive Observation Techniques: Urin din Jubai Soaring Gull did not have the capability to accurately measure the Solar Year, and/or did not consider that the Solar Year and Tropical Year are the same. According to the measurements he did make, 365.35 days seemed the most accurate.

#4 - Calendar modified by the ignorant: If All Soul's Day had been incorrectly added by somebody ignorant in the various science fields, then the original calendar would have been the exact same as the Julian Calendar of our age, with only a divergence of 7.8 days in the 1000 years of the New Era.

Analysis of Possible Explanations

Jordan's oversight: Simplest explanation. However, with a nod to Occam's Razor, I find this answer unsatisfactory.

Different Solar Year: Remotely possible. It could even be argued that this would add a greater degree of scientific validity to the books. But why bother? It certainly won't add to the epic quality of these books for Jordan to explain all of this, especially now that the end is in sight (talk about crashing and burning).

Primitive Observation Techniques: Unlikely. The Roman Empire had no more capabilities than the Sea Folk. Neither did the ancient Egyptians, nor did the Babylonians, nor the Chinese. Neither did 16th century Europeans. All of these civilizations developed calendars remarkably accurate to the cycles of the moon or the turning of the seasons. It is far-fetched to believe that Soaring Gull, apparently respected in his field, would create a calendar so grossly flawed.

Calendar modified by the ignorant: Likely. Although Soaring Gull developed this calendar, it was Farede, the then-Panarch of Tarabon who promoted it, and it was the unwashed masses who adopted it. Anyone could have monkeyed with it, and nobody would have noticed the divergence for 50 - 200 years, except for those few who also made a serious study. At that point, it would just be accepted and adapted.

Conclusion

The Farede calendar did not contain All Soul's Day. Someone else added it.