Another thing that seems wrong is the first value being pushed to the end position:
# if the first value is at 1AM, move first item to end position
I find it strange to do even on full years because it may introduce a step while the data may be meant to be more or less continuous. For instance, midnight air temperature on two successive new year’s eve may be very different so this move may send a -5°C just after a -5°C, introducing a 10°C step. This must have been discussed already, and I don’t mean to reopen the discussion. I guess the effect of the change has been deemed minimal and this is better than having an hour shift all along.
Anyway, this seems even more wrong if the year is incomplete, as the last value may match the next to last even less. But again, perhaps this is insignificant.
My initial point still stands. From what I gathered, the case of an analysis period shorter than a full year is not supported.
I could share the modifications I made to get it to work, but it wouldn’t be a readily mergeable PR as it breaks a few things in epw.py.