How To Calculate Daylight Saving Time (DST) in Python and Windows Powershell

When the boss ain’t happy, boy to we know about it. There’s never a pat on the back though when things are working swimmingly though is there!?

Queue the need for some Python automation that works and takes away that stress so we can recede back into the cocoon of Systems Engineering. I manage a huge fleet of servers around the world, a large portion of them in Daylight Saving Time (DST) influenced timezones. Thus, a large portion of them with the possibility of rebooting upto an hour outside of the Maintenance Window during the summer months. So we need a system in The Cloud, running in Python, that understands if a server is currently in a timezone that is experiencing DST and sets a tag based on the DST aware (corrected) UTC Offset, again in the Powershell example above, this would mean a function that returns +1 and not 0.

Now, there are several methods to fetch timezone from a server. Firstly, we could perform this logic on the server using Powershell and post to AWS (push) or we could perform the compute in AWS using Python and an additional data source (pull). Computing in AWS is the most logical so that the server is free to perform its actual duties. To do this, AWS needs to be aware of the timezone, we gather this via an alternative process (outside the scope of this article) and store it in a separate database, we’ll call this TimezoneInfo.

The timezone, from the pytz module in Python, is provided in the Unix format, for example Europe/London. This can then be used as demonstrated in the below function get_utc_offset This will output a list of all timezones with the corrected UTC Offset for Daylight Saving Time (DST) affected timezones. You could refactor this easily to pass in a single timezone name by removing the for loop, for example, if you had a server name and performed a lookup for the timezone info on another database.

This is How To Calculate Daylight Saving Time (DST) in Python:

import pytz
from datetime import datetime, timedelta

def get_utc_offset(date_time: datetime) -> dict:
    '''
    Gets the UTC offset for a date time.
    '''

    dst_timedelta = date_time.dst() # Returns a timedelta indicating DST
    
    difference_offset = int(dst_timedelta.seconds / 60 / 60)
    offset_dst_aware = int(date_time.utcoffset().total_seconds() / 60 / 60)
    
    # Initialize assuming dst is not set
    date_time_dst = date_time
    offset_dst_unaware = int(date_time.utcoffset().total_seconds() / 60 / 60)
    
    # If the DST timedelta is greater than 0, make adjustments
    if dst_timedelta:
        offset_dst_unaware = offset_dst_unaware - difference_offset
        date_time_dst = date_time - dst_timedelta

    print(f'date_time_utc:         {datetime.utcnow()}')
    print(f'date_time_dst_aware:   {date_time}')
    print(f'date_time_dst_unaware: {date_time_dst}')
    print(f'dst_timedelta:         {dst_timedelta}')
    print(f'offset_dst_aware:      {offset_dst_aware}')
    print(f'offset_dst_unaware:    {offset_dst_unaware}')
    
    return {
        'utc_offset_dst_aware': offset_dst_aware,
        'utc_offset_dst_unaware': offset_dst_unaware
    }

timezones = pytz.all_timezones

for tz_name in timezones:
    print(f'\n{tz_name}')

    tz_info = pytz.timezone(tz_name)
    tz_date = datetime.now(tz_info)
    
    utc_offset = get_utc_offset(tz_date)

'''
# Print Output
Europe/London
date_time_utc:         2024-06-18 16:16:07.073615
date_time_dst_aware:   2024-06-18 17:16:07.073571+01:00
date_time_dst_unaware: 2024-06-18 16:16:07.073571+01:00
dst_timedelta:         1:00:00
offset_dst_aware:      1
offset_dst_unaware:    0

# Object Returned
{'utc_offset_dst_aware': 1, 'utc_offset_dst_unaware': 0}
'''

Or if you don’t care about the DST Unaware, this can be simplified

import pytz
from datetime import datetime, timedelta

def get_utc_offset(date_time: datetime) -> int:
    '''
    Gets the UTC offset for a date time.
    '''

    print('Getting UTC offset')
    utc_offset = int(date_time.utcoffset().total_seconds() / 60 / 60)

    return utc_offset

To use this data, use the returned object to get the utc_offset_dst_aware value which will either be a positive or a negative value. This is your Timezone offset from UTC that is Daylight Saving Time (DST) aware.