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Adjust AWS Cron Events for Daylight Savings Time

Adjust AWS Cron Events using Lambda to minimize Daylight Savings Time's effects on your services.

Posted: April 21, 2020
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In order to lower costs for unused resources in AWS (which add up way faster than you think), we spin-down servers in our non-production environments every night during non-working hours and bring them back up in the morning. This entire process is handled by a single lambda function that is kicked off by some cron triggers in CloudWatch Event rules.

We have the environments set to sleep every weekday between 7pm and 7am, so they’ll be back up in time for the first developers who get in at 7:30am. They also sleep all weekend since we don’t do any development on Saturday or Sunday. Things were working flawlessly! That is, until Daylight Savings Time hit.

The Problem

For those who may not be familiar with Daylight Savings Time (DST), it’s the practice of setting the clocks forward one hour in spring and back one hour in fall to better take advantage of the daylight hours. Unfortunately, since the cron triggers in AWS are set up on UTC time, the DST time change caused our spin-up function to run an hour late.

/* Original CRON triggers when not in DST */
0 13 ? * MON-FRI * // spin-up at 13:00 UTC (7am CST)
0 1 ? * TUE-SAT * // spin-down at 1:00 UTC (7pm CST)

Curious about how cron events work? Cron expressions in AWS are made up of 6 slots. From left to right:

  • Minutes (0-59)
  • Hours (0-23)
  • Day of the Month (0-31)
  • Month (1-12 or JAN-DEC)
  • Day of the Week (1-7 or SUN-SAT)
  • Year (1970-2199)

An asterisk can stand in as a wildcard in any slot.

I adjusted our cron events to account for the time change as follows:

/* New CRON triggers during DST */
0 12 ? * MON-FRI * // spin-up at 12:00 GMT (7am CST)
0 0 ? * TUE-SAT * // spin-down at 0:00 GMT (7pm CST)

This would work, but it was a short term fix. I wanted a longer term solution so we wouldn’t run into this problem again in November when the time changed back.

The Plan

It occurred to me that I could have the very same spin-up/spin-down function adjust its own cron triggers based on whether or not we were in Daylight Savings Time. To do this, I would just need to add a little bit of time logic.

First, I needed to get the current time local time and determine whether or not it was Daylight Savings Time. Thankfully, Python has some very useful date functionality that would make this easy to do.

from datetime import datetime
import pytz

utc_time = datetime.utcnow().replace(tzinfo=pytz.utc)
cst_timezone = pytz.timezone('US/Central')
cst_time = utc_time.astimezone(cst_timezone)

We import the py~datetime library, the main Python library for working with any date or time objects, and py~pytz, a library for accurately handling timezones. We create py~utc_time and set it equal to the current time in UTC with py~datetime.utcnow(). Note that we have to manually replace the timezone information on the native py~datetime object with the py~utc timezone from py~pytz.

Next, we create a central time timezone object py~cst_timezone and pull that information from the py~pytz library. Finally, we convert our UTC time to CST time using the function py~astimezone(), which takes in a timezone object as a parameter.

So now that we have our current time in the CST timezone, we can determine if it’s Daylight Savings Time.

# This will show the offset in seconds for DST ⏰
# If it's zero, we're not in DST
is_dst = cst_time.tzinfo._dst.seconds != 0

If we dig down into our current time object py~cst_time, we can get more information about the timezone. By accessing our timezone information (py~tzinfo), then our DST information (py~_dst), we can find how many py~seconds of an offset is currently on our time. If we are in DST, the offset will not be 0 (it will be 3600 seconds, really, since time gets shifted an hour).

The Triggers

Now that we know if it’s Daylight Savings Time or not, we can adjust our cron triggers. First, let’s define what our expressions will be based on if we’re in DST. During DST, we’ll want the earlier triggers.

if is_dst:
  start_schedule = 'cron(0 12 ? * * *)'
  stop_schedule = 'cron(0 0 ? * * *)'
else:
  start_schedule = 'cron(0 13 ? * * *)'
  stop_schedule = 'cron(0 1 ? * * *)'
# Curious why these cron expressions don't have the weekdays anymore? Read on!

Next, we need to actually update the rules in CloudWatch.

  import boto3
  client = boto3.client('events')

  START_RULE = "start_rule"
  STOP_RULE = "stop_rule"

  start_settings = client.list_targets_by_rule(Rule=START_RULE)
  stop_settings = client.list_targets_by_rule(Rule=STOP_RULE)

We create our py~boto3 client to be able to access our AWS resources. Assuming our rules are named py~"start_rule" and py~"stop_rule", we can then fetch information about these rules from CloudWatch. Here, we are using the function py~list_targets_by_rule() to get back the targets these rules have (that would be this function).

# Remove targets from current rules
client.remove_targets(
  Rule=START_RULE,
  Ids=[(start_settings['Targets'][0]['Id'])]
)
client.remove_targets(
  Rule=STOP_RULE,
  Ids=[(stop_settings['Targets'][0]['Id'])]
)
# Delete rules
client.delete_rule(Name=START_RULE)
client.delete_rule(Name=STOP_RULE)

Technically here we aren’t updating the cron triggers, but rather replacing them. In order to delete a cron event trigger in AWS, you must first remove its targets by passing in an array of IDs. We’ll give it the information we got from our previous call. We only have one target, so we pull the first (and only) item from our settings that we queried like py~start_settings['Targets'][0]['Id'] . Once we delete our targets, we can delete our rules.

# Add new rules
client.put_rule(
  Name=START_RULE,
  ScheduleExpression=start_schedule,
  State='ENABLED',
  Description="Automatic trigger for the Spinup-Spindown function."
)
client.put_rule(
  Name=STOP_RULE,
  ScheduleExpression=stop_schedule,
  State='ENABLED',
  Description="Automatic trigger for the Spinup-Spindown function"
)

Now, we can add our new cron rules in. We’ll set the same name as before and set our new cron expressions that we defined as py~start_schedule and py~stop_schedule. We enable the rule, and add our description.

# Add targets
client.put_targets(
  Rule=START_RULE,
  Targets=[
    {
      'Id': start_settings['Targets'][0]['Id'],
      'Arn':start_settings['Targets'][0]['Arn'],
      'Input': start_settings['Targets'][0]['Input']
    }
  ]
)
client.put_targets(
  Rule=STOP_RULE,
  Targets=[
    {
      'Id': stop_settings['Targets'][0]['Id'],
      'Arn':stop_settings['Targets'][0]['Arn'],
      'Input': stop_settings['Targets'][0]['Input']
    }
  ]
)

Finally, we add our targets back in. We can do this by pulling all the information out of our settings objects. Our updated cron rules are in place!

Final Adjustments

So now our function can update its own cron triggers, awesome! But wait; what happens on the days that the time change actually happens? Won’t our function run an hour late or an hour early on that one particular day? It will indeed, but we come out lucky here.

If you remember from the cron expressions we had at the beginning, we only ran this function Monday through Friday because we don’t want our environments up on the weekend. In the United States, DST begins the 2nd Sunday in March and ends on the 1st Sunday in November. So our 1 hour shift would happen on a weekend, and it would never affect us!

Like I said though, our orignal cron expressions don’t run on weekends, so we’ll need to change our cron expressions slightly so they will. If you notice from the code snippet earlier, we already did this:

if is_dst:
  start_schedule = 'cron(0 12 ? * * *)'
  stop_schedule = 'cron(0 0 ? * * *)'
else:
  start_schedule = 'cron(0 13 ? * * *)'
  stop_schedule = 'cron(0 1 ? * * *)'

These cron expressions will now run every day of the week, instead of just Monday through Friday.

Now that our function runs every day, we’ll need to stop it from doing the spin-up and spin-down on weekends with a bit more time logic.

weekday = cst_time.weekday()
#in the python calendar, 5 and 6 correspond to Saturday and Sunday
if weekday == 5 or weekday == 6:
  return {
      'statusCode': 200,
      'message': 'Today is a not a weekday, so the spin-up/spin-down process will not proceed.'
  }
else:
  begin_startstop_process()

We take our py~cst_time object and pull the day of the week out using py~weekday(). We then check whether it’s a Saturday or a Sunday, and if so, we return a statusCode and message and stop execution. Otherwise we can get things going!

Final Thoughts

So there we go, we can now automatically adjust our cron triggers in AWS so that Daylight Savings Time won’t affect us again! However, there are definitely some future improvements we could make. For example, you might ask the following questions:

Why do we adjust the rules every single day even when the rules will only change twice a year?

What if we have a function that needs to run on weekends that we don’t want to be late twice a year?

This could be resolved pretty simply by extracting all of this functionality into a separate function. Using cron expressions, we could also set this function to only run on Sundays in March and November when the time changes would happen! Let’s wrap this all up (or checkout the gist!):

/* CRON triggers for DST function */
0 12 8-14 MAR SUN * // 13:00 UTC (6am CST) for the second Sunday in March
0 0 1-7 NOV SUN * // 1:00 UTC (6pm CST) for the first Sunday in November
from datetime import datetime
import pytz
import boto3

# define our CloudEvents client
client = boto3.client('events')

# Let's grab our current time in GMT and convert it to local time (US Central)
utc_time = datetime.utcnow().replace(tzinfo=pytz.utc)
cst_timezone = pytz.timezone('US/Central')
cst_time = utc_time.astimezone(cst_timezone)

# This will show the offset in seconds for DST ⏰
# If it's zero, we're not in DST
is_dst = cst_time.tzinfo._dst.seconds != 0

# Determine our new cron expressions for a function running everyday at 7am and 7pm CST
if is_dst:
  start_schedule = 'cron(0 12 ? * * *)'
  stop_schedule = 'cron(0 0 ? * * *)'
else:
  start_schedule = 'cron(0 13 ? * * *)'
  stop_schedule = 'cron(0 1 ? * * *)'

START_RULE = "start_rule"
STOP_RULE = "stop_rule"

# Fetch the targets of our current cron rules
start_settings = client.list_targets_by_rule(Rule=START_RULE)
stop_settings = client.list_targets_by_rule(Rule=STOP_RULE)

# Remove targets from current rules
client.remove_targets(
  Rule=START_RULE,
  Ids=[(start_settings['Targets'][0]['Id'])]
)
client.remove_targets(
  Rule=STOP_RULE,
  Ids=[(stop_settings['Targets'][0]['Id'])]
)

# Delete rules
client.delete_rule(Name=START_RULE)
client.delete_rule(Name=STOP_RULE)

# Add new rules
client.put_rule(
  Name=START_RULE,
  ScheduleExpression=start_schedule,
  State='ENABLED',
  Description="Automatic trigger for the function."
)
client.put_rule(
  Name=STOP_RULE,
  ScheduleExpression=stop_schedule,
  State='ENABLED',
  Description="Automatic trigger for the function"
)

# Add back our targets
client.put_targets(
  Rule=START_RULE,
  Targets=[
    {
      'Id': start_settings['Targets'][0]['Id'],
      'Arn':start_settings['Targets'][0]['Arn'],
      'Input': start_settings['Targets'][0]['Input']
    }
  ]
)
client.put_targets(
  Rule=STOP_RULE,
  Targets=[
    {
      'Id': stop_settings['Targets'][0]['Id'],
      'Arn':stop_settings['Targets'][0]['Arn'],
      'Input': stop_settings['Targets'][0]['Input']
    }
  ]
)

Thanks for sticking around and reading. Hopefully this will help you out with your work in AWS!

Want to dig a bit deeper and learn a bit more? Here are some useful links:

Daylight Savings Time in the US | Wikipedia

Schedule Expressions for Rules | Amazon Web Services

Quick and Simple Cron Expression Editor | crontab guru