Well, in that case, the only real suggestion I have is to downgrade the firmware back to a version that supports manual mode (2.0.9 or below). Since all the new features don’t matter in a setting where the controller gets sent instructions to turn on or off I think that should be okay.
You could also just append the largest value possible to the time value (18 hours for 2.1.3) and let it run until the OpenHAB instructions OpenSprinkler to turn the station off. Although the station would turn off after 18 hours regardless, I hope this would cover the majority of durations needed.