This is not a bug in aCalendar, but a limitation of the sync adapter.
If you use Google Calendar, old events (older than 12 months) are usually only available online in a web browser.
To get older events to sync to Android follow these steps (and repeat every 12 months):
- Go to Google Calendar with a desktop/PC web browser
- export your calendars
- re-import the calendars (needs to be unzipped before)
The re-import will update the modified timestamp which is used by Google to determine if the event should be synced to mobile devices or not.
Unfortunately, we are not aware of such a solution for future events.