use date_to_datetime to prevent always getting date offsets west of UTC

Because of the timezone handling by the clients, using the date directly makes the
min_date on stock moves always end up the day before for negative UTC offsets, since the
date is set to midnight, and then shown with an offset ends up the day before.

This causes us, at UTC-5, to have move and delivery orders planned at 19:00 the day before
the sale order expected date.