one issue sometimes missed is vesting.
you never prorate hours, always use a 12 month period.
so some hours from one year get counted a second time
Labor Reg 2530.203-2©
... For example, a plan which has been using a calendar year vesting computation period is amended to provide for a July 1-June 30 vesting computation period starting in 1977. Employees who complete more than 1,000 hours of service in both of the 12-month periods extending from January 1, 1977 to December 31, 1977 and from July 1, 1977 to June 30, 1978 are advanced two years on the plan's vesting schedule. The plan is deemed to meet the requirements of this subparagraph.