computing the mean for python datetime

You can take the mean of Timedelta. So find the minimum value and subtract it from the series to get a series of Timedelta. Then take the mean and add it back to the minimum.

dob = df_test.DOB
m = dob.min()
(m + (dob - m).mean()).to_pydatetime()

datetime.datetime(2014, 7, 12, 0, 0)

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One-line

df_test.DOB.pipe(lambda d: (lambda m: m + (d - m).mean())(d.min())).to_pydatetime()

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To @ALollz point

I use the epoch pd.Timestamp(0) instead of min

df_test.DOB.pipe(lambda d: (lambda m: m + (d - m).mean())(pd.Timestamp(0))).to_pydatetime()