Everything that makes MailChimp work behind the scenes (sending servers, datacenters, etc) runs based off of UTC (Coordinated Universal Time). However, because you can set your account to display all times in your time zone to make things easier to read while working, it's important to understand that our engines will still refer to any and all timestamps for campaigns or subscriber signups relative to UTC. Depending on where your time zone lies in relation to the International Date Line, subscribers that signed up on the same day in your time zone, may be on different dates relative to UTC because of the time of day they signed up.
How does this affect my Autoresponders?
Autoresponders are unique in the sense that they send based on specific date and time information. When a "signup" date is used as a trigger for an autoresponder, we look into our database to see when a user signed up and we use that date and time to send the autoresponder. This is when all that UTC vs. default time zone information comes into play. Timezones that are after UTC will see that users subscribing after a certain time get their autoresponders a day later than expected.
In the above example list, we have two subscribers who signed up. Subscriber 1 has a date added of 07/18/2011 6:00am EST and Subscriber2 has a date added of 07/18/2011 and a time of 10:00pm EST.
- The subscriber "Cindy Chimp" has a signup date of 7/19/2011 6:34 am EST which translates to 7/19/2011 10:34 am UTC.
- The subscriber "Sniffles Newbie" has a signup date of 7/19/2011 9:35 pm EST which translates to 7/20/2011 1:35 am UTC
An autoresponder is set up to send on "day after signup".
- On 07/20/2011 only Subscriber 1 receives the autoresponder because their UTC "Date Added" timestamp is 07/19 UTC.
- On 07/21/2011 only Subscriber 2 receives the autoresponder because their UTC "Date Added" timestamp is 07/20 UTC.
Both subscribers still receive the autoresponder as they should.
None of your subscribers will ever be completely skipped from an autoresponder. Autoresponders will send to all subscribers who meet the criteria specified by your send settings. This example is only here to explain what is going on if you see someone receive an autoresponder a day later than you, or they, may have been expecting.
Does this affect my other campaigns?
Nope! It doesn't. Not in any way shape or form, this only relates to autoresponders triggering off of a "signup" field.