Python class to determine if a datetime string occurred during STD or DST

is_datetime_std_or_dst.py

#!/usr/bin/env python3

import datetime as dt
import subprocess

class STD_DST(dt.tzinfo):
    """ Determine if a datetime occured during STD or DST time """
    def dst(self, a):
        # DST starts second Sunday in March at 2AM
        d = dt.datetime(a.year, 3, 1, 2)
        self.dston = d + dt.timedelta(days=14-(d.weekday()+1))
        # DST ends first Sunday in November at 2AM
        d = dt.datetime(a.year, 11, 1, 2)
        self.dstoff = d + dt.timedelta(days=7-(d.weekday()+1))
        if self.dston <= a.replace(tzinfo=None) < self.dstoff:
            return dt.timedelta(hours=1)
        else:
            return dt.timedelta(0)
    def tzname(self,a):
        if self.dst(a):
            return("DST")
        else:
            return("STD")
    def utcoffset(self, a):
        # Use any date that's in STD time to get the standard offset from UTC.
        # That's 5 hours for EST.
        cmd = subprocess.run(["date", "-d@1451624400", "+%:z"],
                             stdout=subprocess.PIPE)
        std_offset = int(cmd.stdout.decode().split(':')[0][1:])
        # Return the time difference between a and UTC.
        return dt.timedelta(hours=std_offset) - self.dst(a)

if __name__ == "__main__":
    std_dst = STD_DST()

    assert(dt.datetime(2016,3,13,1,59,0,tzinfo=std_dst).tzname() == 'STD')
    assert(dt.datetime(2016,3,13,2,0,0,tzinfo=std_dst).tzname() == 'DST')
    assert(dt.datetime(2016,11,6,1,59,0,tzinfo=std_dst).tzname() == 'DST')
    assert(dt.datetime(2016,11,6,2,0,0,tzinfo=std_dst).tzname() == 'STD')
    assert(dt.datetime(2016,3,13,1,59,0,tzinfo=std_dst).utcoffset()
           == dt.timedelta(hours=5))
    assert(dt.datetime(2016,3,13,2,0,0,tzinfo=std_dst).utcoffset()
           == dt.timedelta(hours=4))
    assert(dt.datetime(2016,11,6,1,59,0,tzinfo=std_dst).utcoffset()
           == dt.timedelta(hours=4))
    assert(dt.datetime(2016,11,6,2,0,0,tzinfo=std_dst).utcoffset()
           == dt.timedelta(hours=5))