Dates and Times in Python: average of two dates with Pandas

I spent a little longer than expected figuring out how to find the midpoint of two dates for a little table of data recently. Here is a code snippet in case I (or you) have to do this again:

# midpoint of two date columns
df = pd.DataFrame({'a': ['5/1/2012 0:00', '4/1/2014 0:00'],
                   'b': ['4/1/2014 0:00', 'unknown']})

# make time data into Timestamp format
def try_totime(t):
    try:
        return pd.Timestamp(t)
    except:
        return np.nan
    
df['start'] = df.a.map(try_totime)
df['end'] = df.b.map(try_totime)

# generate midpoint time
# harder than it would seem...
df['time'] = df.start + (df.end - df.start)/2

df