We do know how long an Eorzean Year is. Every month has 32 days in it and there are 12 months in an Eorzean Year. So an Eorzean Year is 384 days long. The length of the months are tied to the movements of the moon.

The timeline is not a mess. There is a timeline of the 6th Astral Era in Lore Book 1 that gives years for the major events and the timeline is around four pages long. If anything, FFXIV has an unusually well-put together timeline compared to most games with lore this developed.