Converting time string

def appointment_time_string(time_str):
    import datetime
    a = time_str.split()[0]

    in_time = datetime.datetime.strptime(a,'%I:%M%p')

    start_time = str(datetime.datetime.strftime(in_time, "%H:%M:%S")) + "Z"

    if time_str.split()[3] == 'Today,':
        start_date = datetime.datetime.utcnow().strftime("%Y-%m-%dT")
    elif time_str.split()[3] == 'Tomorrow,':
        today = datetime.date.today( )
        start_date = (today + datetime.timedelta(days=1)).strftime("%Y-%m-%dT")

    appointment_time = str(start_date) + str(start_time)

    return appointment_time