Is bonus calculated from the date of joining of an employee or from the date of receiving of confirmation letter (which may be 6 / 9 months after receiving of joining letter)

Bonus for an employee is calculated from the date the employee assumed the position.
The bonus eligibility letter should be sent at that time to. In fact, in my job (Comp&Ben Analyst) the bonus eligibility letter (containing a summury of how the bonus will be calculateded and from what factors i.e.. Financial performance or personal objectives) is sent along with the job contract that the employee signs.
Hope that helped...

A bonus is calculated according to the policy under which the bonus is paid. Eligibilty and amount will vary depending on the terms used by your company. Therefore we can't answer that question without knowing the company policy.

The bonus is calculated from the day one when an employee occupies his/her position in the company