Some cloud services have limitations in syncing past / future events. Sometimes, you're lucky and there is a setting expanding the time period.

aCalendar itself is just a user interface to the Android calendar storage and does not save any calendar data or is involved in synchronization.

For Google Calendars: 

This is a limitation of Google's calendar sync on Android. Events older than 12 months (and events more than 12 months into the future) are usually only available online in the web browser at Google Calendar.

The Google Calendar app seems to be experimenting with extended but proprietary sync - unfortunately, the data is locked in and not shared with other calendar apps like aCalendar.

Experimental Solution (requires aCalendar+ or Business Features)

You can set the future sync time window to 1, 3 or 5 years and you can request re-sync for the events of the past 3 calendar years. This is offered in the calendar list (MENU - Manage Calendars) in the 3-dots menu for each Google Calendar (though the future sync time window is actually set per account).

Manual workaround (past events only):

To manually synchronize old events to Android, you can use the following workaround (needs to be repeated every 12 months):

  1. Go to Google Calendar on a PC in a web browser
  2. export your calendars (Export your Google calendar)
  3. re-import the calendars (needs to be unzipped before) (Import events to Google Calendar)

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.