I remember reading in my Roman history classes that Julius Caesar had to modify the calender after he became dictator to get it into alignment with the seasons, and further modification was needed in later centuries to make it the 365 1/4th calender we have today.
How accurate were calenders in eastern civilizations? What did Chinese, Koreans and Japanese use, and were they forced to adjust it multiple times?
Ugh, I tried to do some research on this and got bombarded with so much math. My brain hurts.
The Chinese used a Lunisolar calender, which in contrast to the Julian and Gregorian calendar, uses both the Sun and the Moon as time markers.
The Sifen Calendar was created in the Spring and Autumn Period, around 500 BC. It measured a year as 365 and 1/4 days. In contrast the Julian Calendar, which also measured a year as 365 and 1/4 days, was created in 46 BC.
This was replaced with the Taichu Calendar in 104 BC, which measured a year as 365 and 385/1539 days.
Two centuries later the Chinese reverted to the Sifen Calendar, because the Taichu had become inaccurate over the centuries.
The Shoushi Calendar was created in 1281 AD. It measured a year as 365 and .2425 days. In contrast the Gregorian Calendar, which uses the same measurement, was created in 1582 AD.
Japan and Korea both used calendars that derived from Chinese calendars, with their own additions and changes.
Also, before the Sifen Calendar the Chinese used the Sexgenary Cycle, which had two cycles. The Heavenly Stems, which had 10 units, and the Earthly Branches, which had 12 units. There's some math involved, and I will expand on if I can understand it. If not I suggest you ask a mathematician.
Since you asked about Eastern calenders, Here is wiki link for Indian calenders
To make it short
Concept of time is not linear in Indian. Cyclical(No start, No end) times are divided linearly into 4 Yugas, (After completion of one cycle i e 4 yugas universe will destroy itself and recreates itself ). Time is linear in yugas.
calenders are based on lunisolar.
There are many calenders derived from Vedic period, Modern India and south east asian calenders are based on these.
Each years has 12 months and 7 days week. but 356 days and every 3 years extra month is added .
Each years has name numbering 60, after every 60 years (mini)cycle starts with many such cycle numbering to million or billion years( with lot hindu religion intervened in between )
Wiki article has more elaborate explanation with many graphical illustrations