In voyage chartering, permitting charterers to add up the laytime allowed for loading and discharging is called Reversible Laytime. In other words, any time saved (or exceeded) at the load port can be carried forward to the discharge port. Depending on the drafting of the relevant clause, charterers may only have a limited period of time for deciding whether to reverse.
In Reversible Laytime, excepted days must not be added to (or subtracted from) the laytime. For example, laytime for loading is five (5) weather working days of 24 consecutive hours. Sundays are excepted (SHEX). If laytime commences on a Thursday and the ship completes loading on Saturday, then in the normal course of events, laytime at the load port would have expired on the Tuesday. In theory, by being able to depart on the Saturday, ship has saved three (3) calendar days. However, three (3) days may not be transferred to the time available to be used at the discharge port. Days which have been saved at the load port are the Monday and the Tuesday only. On this example, only two (2) days can be added to the time available at the discharge port.
In opposite case, where time has been exceeded at the load port. Time allowed at the load port is five (5) days, if laytime were to commence on Monday, laytime would expire on Friday. If loading was not completed until the following Monday, the excess time of three (3) calendar days would have been used and the ship would have been on demurrage for three (3) days. However, if laytime were to be reversed in this case, only two (2) days (Saturday and Monday) would have to be borrowed from the time allowed for discharging.
Although Average Laytime and Reversible Laytime have the same objective, the outcome will be different according to whether the charter-party allows the charterers to average or to reverse the laytime.