Project worksheet says "dates cannot be changed in past periods" when only changing planned hours/days (CP2012)

Symptoms:
When changing the planned hours/days for an existing assignment, Changepoint says that the planned start or planned finish cannot be moved to a past fiscal period. However:

1- we are not changing the planned start or end date
2- there are no time entries outside the assignment duration that would cause Changepoint to change the planned start
3- there are no task dependencies


Error/warning messages:
Changes to planned start date or planned finish date in a past fiscal period is not allowed.

Reason:

Assignment data was imported from outside of Changepoint, creating assignments with planned start dates containing time portion (e.g. 2010-01-19 10:39:39.037).

Planned start/finish and forecast start/finish dates in TaskAssignment table should not have the time portion (e.g. 2010-01-19 00:00:00.000).