Python convert long to date

I'm trying to convert a long to a date:

class timeStamp(object):
     def getDateTime(self,longDate):                                                                                                                                 
         myNumber = float(longDate)
         return str(datetime.datetime.fromtimestamp(time.ctime(myNumber)).strftime('%Y-%m-%d %H:%M:%S'))

But I have a weird error:

 File "./index.py", line 104, in getDateTime
    return str(datetime.datetime.fromtimestamp(time.ctime(myNumber)).strftime('%Y-%m-%d %H:%M:%S'))
TypeError: a float is required

Why is it complaining when I explicitly cast it to float? The long is a Unix timestamp stored as a long in mysql.