You have a Text object. The strftime function requires a datetime object. The code below takes an intermediate step of converting your Text to a datetime using strptime
import datetime
testeddate="4/25/2015"
dt_obj = datetime.datetime.strptime(testeddate,'%m/%d/%Y')
At this point, the dt_obj is a datetime object. This means we can easily convert it to a string with any format. In your particular case:
dt_str = datetime.datetime.strftime(dt_obj,'%Y-%m-%d %H:%M:%S')
The dt_str now is:
'2015-04-25 00:00:00'