Dates imported from MS-Excel are off by 4 years
You can import data from an MS-Excel spreadsheet into Recurring Holidays, One-Time-Dates and Slogans (use the corresponding links for more details). Sometimes, however, if the Excel file was created on the Macintosh, you will find that when you open that spreadsheet under Windows, dates are approximately 4 years off from the values they had on the Macintosh.
This is due to the setting 1904 date system in the Macintosh version of MS-Excel (see above). Always make sure that your version of Excel has this option un-checked.