Currently day_offset in the API uses days defined by UTC rather than the local time of the station. This makes it relatively useless for any station that is not in the UTC time zone.
For example, say I want to calculate rain accumulation at a specific station since midnight in local time (say UTC-3). Common sense would suggest I can use day_offset=0 to get all data since midnight (i.e. the current day). That won’t work however, as day_offset=0 will return data from 21:00 the previous day in local time, as that is midnight UTC for the current day.