According to which calendar should we calculate the age of an animal?

Could an animal born on Eid al-Adha be slaughtered as sacrifice after two years? Has he then reached the age of two? According to which calendar should we calculate the age of an animal, the hijri calendar or the gregorian calendar?

The age of an animal should be calculated according to the lunar calendar. This is 354 days.  An animal, such as cattle born on eid al-adha would reach the age for sacrifice after 2 years (on eid al-adha). Thereby it can be sacrificed.

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Date: Oct 19, 2011