I think it might just be a problem with trying to figure out sensible defaults when going from a date to a date-time

and I've got start/end dates and the swap from one interval to another happens at some point on the end of the old to the start of the new

It looks like this is because this is what new-interval is doing:

{:tick/beginning (t/beginning (t/bounds (t/date "2020-01-01")))
 :tick/end       (t/end (t/bounds (t/date "2020-01-02")))}

@danvingo yes, that looks right to me. So I just need to tweak the beginning and end dates if I'm only passing in dates. I wonder if there should be different behaviour if an interval is only made of dates rather than date times

I don't think bounds works for date-times because ITimeSpan just returns the date-time itself for beginning and end:

(t/beginning (t/date-time "2020-01-02T00:00"))
=> #time/date-time"2020-01-02T00:00"
(t/end (t/date-time "2020-01-02T00:00"))
=> #time/date-time"2020-01-02T00:00"