This function calculates week day according to 1900 leap year bug, described in observations to Part 3, section 3.16.9.1 and Part 4, section 3.17.4.1. A software error should be corrected, not documented in a standard.

To adequate function description to a date representation that conforms to ISO 8601.

Weekday’ function Part 4, section 3.17.7.341

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Proposed Disposition of DIS 29500 Comment CL-0015 (Modified: 2008-01-02) Agreed; what was formerly the 1900 date base in SpreadsheetML - now called the 1900 backwards compatibility date base as a response to BR-0046, CH-0006 (and more) - that several functions dealing with dates and durations would incorrectly calculate those durations across February 28, 1900 and March 1, 1900. While that calculation bug must be maintained by these functions for compatibility with the existing corpus of binary documents, we do recognize the importance of fixing this calculation bug. As such, we have created a new date base as a response to BR-0046, CH-006 (and more) that both fixes the 1900 leap year bug and supports dates prior to the year 1900. When a SpreadsheetML document uses this new 1900 date base, functions such as WEEKDAY that deal with dates and durations should no longer display the buggy behaviour. Because the definitions of these functions already refer to §3.17.4.1 which describes the date base systems in SpreadsheetML, no update is necessary to §3.17.7.341 for this disposition. Similar Comments: CA-0071 , CL-0172 , DE-0073 , FR-0352 , GB-0305 , GR-0007 , IN-0062 , US-0134