1900ndate

1900ndate is a term that appears in informal discussions to denote a date encoding scheme based on a fixed origin at January 1, 1900. It is not an official standard, and its exact meaning can vary by context. In many cases, 1900ndate refers to representing dates as the number of days elapsed since 1900-01-01, a concept also described as a serial date or Excel-style date.

Origins and variations of the concept stem from widespread use of year-1900 baselines in legacy software and data formats. The most familiar example is the Excel 1900 date system used on Windows, where dates are stored as integers counting days from 1900-01-01, including a known anomaly that 1900 was treated as a leap year (creating a phantom 1900-02-29). Other environments, such as some Mac systems, use a 1904-based baseline, which shifts dates by several years when converting between systems. These differences illustrate why the exact definition of 1900ndate can vary.

Common uses and implications include data migration, historical archiving, and software development where compact date storage or simple arithmetic on dates is desirable. Practitioners should document the exact base date, whether the leap-year anomaly is present, and how time zones are handled when converting to or from ISO 8601 or human-readable forms. Misunderstandings about the base can lead to off-by-one-day errors or inconsistent date representations across systems.

See also: serial date, Excel date system, ISO 8601, Unix time, date baselines.

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