According to the documentation there are two values it can take. Period and schedule. I can’t really find any information about what the differences between these two are.
This schedule option is currently missing due to some issue with this option. However, it should be added with v7.5 firmware version. If everything goes well, v7.5 should be released soon. You will be able to specify time in terms of hours and minutes, when the configured Modbus requests should be sent.