MMC maintains a qualified defined benefit plan for eligible employees, with an effective date of January 1, 1990. The plan year for vesting and participation purposes is the calendar year.

Eligibility is age 21 + one year wait + salaried status, with monthly entry dates; Vesting is the 3-to-7 graded schedule, with 1,000 hours required in a plan year.

Harriet was born on November 18, 1957 and hired as a full-time hourly employee of MMC on December 1, 1988. She transferred to full-time salaried status on June 1, 1990. 

What is her entry date into the plan?