This is from Ray on a different post:
Yes, deleting a log file is done via:
where x.x.x.x is your opensprinkler’s ip address, pw is the password, day is the epoch time in days. To find the epoch time, google ‘epoch time’ and divide the long number by 86400 (number of seconds in a day). For exampe, for July 5, 2014 it is 16256.
I know this is a rather awkward way to delete a log file manually, and a better way to delete the log file will be added soon which allows you to select a day in the UI and delete it that way.