I just discovered this the other day:

See: http://www.united.com/CMS/en-US/trav...ellations.aspx under Notification of delays and cancellations

Verification letters

Upon request, we can provide a statement of proof that a United flight was delayed or canceled. You may email your request to [email protected]. Be sure to include the names of all customers in your party, confirmation number, flight numbers and dates of travel, phone contact and the email address or mailing address where you would like it sent. Please allow 5-10 business days for processing.

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Once I was delayed due to weather at Narita for 3 days - upon arrival at Taipei, the gate agents were handing them out. In Taiwan, you need the letter to be properly excused from work or school. The rail system does that too.