Are you referring to https://j-novel.club/calendar ? It's possible that it considers the 4/1 entry to actually be 3/31 depending on your time zone, so it doesn't display it as April.
For me, it shows 4/1 data, but months that don't have entries for the first (like https://j-novel.club/calendar?year=2023&month=1 ) start on the first day with things being released. Yesterday, before today's entry was posted, the April calendar started on 4/3, Monday.