Yearfirst

Yearfirst refers to a parsing preference used by date and time parsers to treat the year component as the first element of a date string. It signals that the string is expected to start with a four-digit year, enabling unambiguous interpretation of formats where the year is first, such as ISO-like or custom datasets. In practice, yearfirst is typically a boolean option in date parsing libraries, controlling how the parser assigns the year, month, and day when the input contains multiple numeric components.

In Python's pandas library, for example, the to_datetime function accepts a yearfirst parameter to influence parsing when the date string begins with a year. When yearfirst is true, strings like "2023-04-05" or "2023/04/05" are interpreted with 2023 as the year, and the remaining components as month and day. If false, parsers may apply regional defaults or attempt other interpretations.

Yearfirst is mainly relevant for data cleaning and ingestion tasks involving heterogeneous date formats, log files, or international datasets where year-first notation is common. It is not a universal standard and its availability and behavior vary by library and locale. Misuse can lead to incorrect dates, especially with two-digit years or nonstandard separators.

For related concepts, see dayfirst, two-digit year handling, and ISO 8601 formats.