Order Release Sequence Advice - Scheduled Ship Date Timestamp 23:59

Hi

Our business release from 18 operating units to a central single shared warehouse, to ensure our order release cycle considers the time stamp of the order line to ensure real fifo of our order releases. What is the correct approach to set the Order Release Sequence ?

Currently is set using Scheduled Ship Date, but this is defaulted to timestamp  23:59 , therefore not providing a fifo control within that single days orders (if multiple order lines for same item within single day, with limited on-hand, what is the control to ensure the earliest order of that date/day is released first if all 23:59)