I saw many posts they do not know what Chinese Zodiac they are.
It is confused by many people do not understand Chinese astrology system.
Even Chinese people will also confuse it.

What Chinese Zodiac you are will be decided by the beginning of Jie Chi/Qi(lì chūn). It is the beginning of Spring.
The day of "Lì Chūn" is usually on Feb. 4th or 5th.

The example,
If you were born after Feb 4th or 5th 2007, your Chinese Zodiac is "Pig".
If you were born after Feb 4th or 5th 2008, your Chinese Zodiac is "Rat".
If you were born before Feb 4th or 5th 2008, your Chinese Zodiac is "Pig".
Rat(2008), Ox(2009), Tiger(2010), Rabbit(2011), Dragon(2012), Snake(2013), Horse(2014), Goat(2015), Monkey(2016), Rooster(2017), Dog(2018), Pig(2019).

If you were born at Feb. 4ht or 5th, you better ask people to help you to find out.

Four Pillars (one of Chinese astrology) is formed by this rule.

Please do not ask me how Chinese Zodiac is formed. I do not know it.  It is a legend.