I’m noticing different values for these two different queries:

SELECT SUM (CASE WHEN date1 - date2 <= INTERVAL '1 year' THEN 1 ELSE 0 END)

SELECT SUM (CASE WHEN date1 <= date2 + INTERVAL '1 year' THEN 1 ELSE 0 END)

with the latter being less restrictive (larger sum).

Both date1 and date2 are Timestamps.