Sweep intervals do not fall on any defined moment in time. If you have chosen to discard historical data, the only guarantee is that records are kept for the specified length of time or specified number of records.
If storage limiting is in effect, then regardless of any property settings, the maximum interval between sweeps will be no greater than 365 days and no less than one hour.
Default: HistorianDataAgeSweepIntervalDivisor = 4
The term "sweep interval" is slightly misleading in that the interval actually sets the time between creation of new record sequences, "blocks", in the Historian. During a sweep, entire record sequences older than the number of days or records to keep, are removed. Changes to the sweep interval do not affect existing record sequences. Sweeps may occur on any of application startup, workstation becomes the server, and again after the next interval since the last sweep.
Sweeping is done based on when data was recorded, which may differ from the data timestamp, especially in the case of manually entered data. Sweeping is based only on the age of the record sequence and is not done on a tag-by-tag basis, nor do sweep intervals line up with calendar dates such as January 1st.
Again, if you have chosen to delete data, the only guarantee is that records newer than the limit are kept and that limit may be set to any length of time. Older records will be swept eventually.