df = pd.DataFrame({'date':['31DEC2002','31 December 2015 00:00:00.000 GMT','.']})
df['date'] = pd.to_datetime(df['date'], utc=True, errors='coerce')
print (df)
date
0 2002-12-31 00:00:00+00:00
1 2015-12-31 00:00:00+00:00
2 NaT