Use Pandas.
import pandas as pd
r = ['01-01-15','02-01-15','03-01-15'] #creating data
df =pd.DataFrame({'date':r})
#add '20' in year
df.date = df.date.apply(lambda x: x[:6]+'20'+x[6:])
#convert to datetime
df.date = pd.to_datetime(df['date'], format='%m-%d-%Y')
hope this helps. Cheers :)