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Join Date: Jun 2008
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Posts: 33,120
The rule is very clear, your ESTA must be valid on the day when you enter the US. Even if it expires before your return (and as long as the return complies with your authorised stay in the US which is usually 3 months) you are fine.
The reminder you receive is automatic.
So if your ESTA is valid till 31/1/2016:
- Arrive on 30/1 leave on 31/1: fine
- Arrive on 30/1 planning to stay till 31/3: fine
- Arrive on 1/2 regardless of your planned stay: not fine