The phrase " as accurate as possible" more implies a hope than a request. It is ambiguously and should come with a precise description, which accuracy is need, Is it fine if it is within the same century, only a 10-year spread or do we require sub-second accuracy
Accuracy usually depends on the context, and the application

Specify the minimum precision requirements

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Section 3.17.6.7

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Proposed Disposition of DIS 29500 Comment DK-0034 (Modified: 2007-11-28) We agree; The following change will be made to Part 4, §3.17.6.7, page 2,529, line 29: As a date and/or time is represented by a number, a date/time serial value shall be stored in XML as the unformatted text form of that number (according to the lexical representation of numeric values given in § 3.17.5.3) , as accurately as possible . Similar Comments: CL-0148 , CO-0158 , FR-0439 , GB-0366 , GB-0369 , GR-0099 , IN-0071 , US-0169