import time
import pandas as pd

date_stngs = ('2008-12-20','2008-12-21','2008-12-22','2008-12-23') * 100000

t = time.time()
pd.Series([pd.to_datetime(date) for date in date_stngs])
print time.time() - t

t = time.time()
pd.Series([pd.to_datetime(date, '%Y-%m-%d') for date in date_stngs])
print time.time() - t