InfoPath dates are stored in the format YYYY-MM-DD as already discussed in Date and time basics in Microsoft Office InfoPath.
The beauty of this format is that you can easily use InfoPath functions, such as substring() and number() to extract the numbers for the day, month, and year from a date .
To extract the day number of a date, use
number(substring(field1, 9, 2))
To extract the month number of a date, use
number(substring(field1, 6, 2))
To extract the year number of a date, use
number(substring(field1, 1, 4))
where field1 represents a date picker control on an InfoPath form template.
Once you have the number, you can do very neat things such as use the number for the month in date together with rules, filters, and a data connection to retrieve the name for the month.
Update (Jan 11, 2009):
The date format used by InfoPath also lends itself well to comparing dates. You can for example easily use this date format together with data validation rules to check whether a start date falls before an end date.
For more InfoPath solutions that make use of dates, see Use Date Picker Controls in InfoPath.
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