This seems a bit hackish, but will work for dates beteen ~ 1970 and 2030 (on 32 bit arch). You are essentially converting the datetime values to integer, averaging them, and converting the average back to a datetime value.
SELECT
from_unixtime(
avg(
unix_timestamp(date_one)-unix_timestamp(date_two)
)
)
FROM
some_table
WHERE
some-restriction-applies
There is likely a better solution out there, but this will get you by in a pinch.
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