In the official Marketo documentation it says "Per Day is based on the calendar day in the subscription time zone (midnight-midnight)".
But in reality the communication limit does not refresh mid night and it rather works in a 24 hour window.
For reference : My customer has set a communication limit as 1 email per day. An email was sent at 10 am yesterday and today at 10 am an email is skipped due to communication limit. I tested this briefly and also discussed this with fellow Tier 3 colleagues and confirmed that the refresh window for communication limit is actually 24 hours and not mid night.
So when you send an email today at 10 am , make sure send another at 11 am the next day.
Hope this helps. Also a suggestion to Marketo to update the documentation on this.
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