Vesting YOS

My client intends that vesting service starts on participation date, not hire date. I referenced the plan document which specifies YOS starting with employment date, however, I cannot find anything that specifically states that participant date cannot be the start date for vesting. Any ideas? 

How about the ERISA code? And what you can and can not exclude. It's pretty much black letter law.

See §1053(b) in the attached link

https://www.law.cornell.edu/uscode/text/29/1053

exactly - must include all employment with noted exceptions. can only start from participation date for those in the plan on the effective date (excluding service before there was a plan).