The both parties in their joint petition for divorce by mutual consent clearly mention the date when they both got separated, in your case that should be 12 months back when you actually separated, that date has to be mentioned. Noone question this date as when it is not being disputed by either of the parties it is taken as correct by the court also.
No other evidence of proof required for the date of separation but joint statement in the court from the date you both started living separatly and are not in relationship of husband and wife.
The complete process is of six months after you first file joint petition for divorce under section 13 B the Hindu Marriage Act,1955 for Hindu marriage dissolution, and first motion where statement of both parties recorded of living separate for more than 12 months and seeking divorce by their free consent, followed by second motion after 6 months when again both parties record their statement of moving ahead with divorce. The court being satisfied allows the divorce and issue decree of divorce to you.
