What is the "Earliest Date on which benefits will becom vested"?

[ERISA1]

ERISA Section 105 requires benefit statement to include a statement of "vested percentage of such benefits (or the earliest date on which benefits will become vested)".  Do you agree that the "earliest date" is the date on which a participant becomes partial vested?  That is, on a 2/20 vesting schedule, does a participant "become vested" when they become 20% vested or when they become 100% vested?

Thanks

[My 2 cents]

Whether required or not, the benefit statement for a plan with graded vesting should disclose not only the date as of which a non-vested participant will attain partial vesting but also the date as of which a non-vested or partially vested participant will attain full vesting.  The same assumption concerning accrual of future vesting service should be used for both purposes (and why not 1 year of vesting credit for each future year on an elapsed time basis?).

Surely, such information would be useful to the participants, so why not give it to them?