You can select seconds up to 59 seconds. The the granularity becomes minutes. The reason for this is to save memory space: because all programmed water times need to be stored in non-volatile memory, and the amount of non-volatile memory is very limited, it’s necessary to find a reasonable compression scheme that uses non-linear scaling to allow a wide range of water times, but at the cost of reduced granularity. In addition, OpenSprinkler firmware supports up to 56 stations and up to 28 programs, so the amount of memory requires to store all the water times can quickly reach the hardware limit. To be honest, most other sprinkler controllers don’t even allow any second-level water time — they all require minutes.
We are already working to address this in the next firmware, so the limitation should be gone relatively soon.