An old Mac may contain a local calendar that needs to be added to a newer setup. Importing a full Calendar archive as a quick test can replace current information. Use the single-calendar event route when the task is adding one calendar’s events.

In Calendar on Mac, show the calendar list and identify the source calendar by account and name. Confirm that it contains the intended events. A colour alone is an unreliable identifier when several calendars look similar.

Export the selected calendar

Select its name and choose File, Export, Export. Apple saves the calendar’s events as an .ics file. Give it a dated name that identifies the source, and keep the original calendar intact.

For comparison, File, Export, Calendar Archive creates an .icbu archive of all calendars. Apple explicitly warns that importing such an archive replaces current calendar information and data. Keep recovery archives separate from files meant for an additive transfer.

Import into a deliberate destination

Create a separate destination calendar if appropriate, then use File, Import for the .ics file and select the calendar that should receive the events. Read the destination choice carefully before confirming. Apple notes that imported events lose their previous custom colours.

Inspect several event types: a timed appointment, an all-day item and a recurring series if the source contains one. Check titles, dates, time zones and notes rather than relying on the number of visible entries in a single week.

Keep the source and destination visually distinguishable while comparing. If both are shown simultaneously, the same event can appear twice even though the import produced one new copy.

Avoid repeatedly importing the same file while investigating a display mismatch. First establish whether the events already exist in the destination, since another import may complicate the comparison.

Record the accepted destination and checks before retiring the source calendar. This is a transfer of saved event data. It does not by itself recreate sharing permissions or establish that another account now owns an ongoing team calendar.

References: Apple: Calendar import and export.