<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def timestamp(datetime_obj):
 """Return Unix timestamp as float. The number of seconds that have elapsed since January 1, 1970. """

    start_of_time = datetime.datetime(1970, 1, 1)
    diff = datetime_obj - start_of_time
    return diff.total_seconds()