Here is a list of Thellid Leap Years from the surrounding 200 years. Note that Thellid Leap Years do not correspond to the customary Gregorian ones. Also note that these years are in Human Era (HE) format which prefixes a "1" to all modern dates. ie 12,000 HE is the Gregorian year 2,000.
It is easy to ascertain which years are Thellid Leap Years from a list of Greenwich Winter Solstice times in GMT standard. These occur when the solstice time first crosses from PM to AM. For example: the GWS in 2001 was at 7:21 PM on 21/12 but in 2002 it occurred at 1:14 AM on 22/12. Therefore 2002 was a Thellid Leap Year.
You will note that Thellid Leap Years mostly progress in jumps of 4 years, but about 3 times a century they jump 5. Each time a Leap Year is pushed back a year you lose ¼ of a leap day. This means that over a century you lose ¾ of a leap day and over 400 years you lose 3 leap days. This is because the solar year is not exactly 365¼ days but actually about 11 minutes shorter than that. Over 400 years this amounts to about 3 days, and these are the days lost by the occasional jump of 5 instead of 4.
The Gregorian Calendar loses 3 leap days over 400 years as well, but these are achieved by a leap year jump of 8 years at century boundaries 3 times out of 4. The Thellid and Gregorian Calendars both have 97 leap years in a 400 year period, so they both end up telling the same time. The difference is that the Thellid Calendar loses its leap days ¼ at a time every 33 years, whilst the Gregorian loses them in one chunk every 100 years, 3 centuries out of 4.
Due to the tidal effects of the orbiting moon and also that of the sun, the Earth's rotational period is gradually slowing. Our day length is increasing by about 1.7 milliseconds a century, (or 0.000017 s/yr). This means that the number of days per year is decreasing by 0.0062 seconds each year. The Thellid Calendar is based on the Tropical Year which had a length of 365.242181 mean solar days in 2010. When the number of mean solar days per tropical year reaches exactly 365 there will no longer be any need for a Leap Day. Divide the current excess seconds per year (0.242*24*60*60 = 20,908) by the slowing per year (0.0062) and you get 3.4 million years. This is the time until we will no longer need the Leap Day.
When this point is reached we can move the Old Year's Day to the last day of Zithebbe and get rid of Werrimul. But 3.4 million years is a very long time in the human scale and no one can predict what might be happening by then...