Zelith seems to be a period of time within Furrae's calendar, possibly the 16th century when Abel was born. Little is known of the time periods other than the fact that they were named along with the year in Furrae's calendar in comic #1193 when M.A.C.E was researching Abel online in Zinvth's public registry. This theory is supported by the fact that the date next to Katith was 1655 (May Rewanz's death at the age of 83-84), the date for Dekith was 1964 (Dekith may be the century in which DMFA canon is set), and the three dates for Zelith were 1571, 1591, and 1598. Adding Abel's age (nearly 400 at the time of writing) to 1591 gives a date of 1991 (Dekith?) for the set date of DMFA. Adding Alexsi's age to 1964 (27, if she was born that year) gives the same conclusion
Given the dates in the referenced comic, it seems that Abel's story lasted from 1591 Zelith to 1655 Katith. A timeline can be made for the story arc with these dates and knowledge of the storyline (1615 being the year Devin and Xander died, Abel became a cubi, Hennya was killed, and Abel began living at SAIA). Fa'Lina and Aniz's ages at that time can also be calculated with 1991 Dekith as the set date of the "present day" (9,288 in 1991 and the age back in 1615, 8,912).