Peruspäivän

Peruspäivän is the genitive form of peruspäivä, a Finnish compound noun formed from perus- meaning "base" or "foundation" and päivän meaning "of day." The compound is not a single standardized term in Finnish; rather, peruspäivän is used in various technical contexts to denote a baseline or reference day—the day from which measurements, calculations, or plans are anchored. Because Finnish allows flexible compounding, the exact meaning of peruspäivän depends on the domain and the accompanying nouns.

In statistics and data analysis, a base day may serve as the reference date for a time series, so that other values are expressed as changes or percentages relative to that day. In project management or operations planning, a baseline day can mark the start of a schedule or the point against which progress is measured.

In human resources or benefits administration, some documents describe a peruspäivä as the day used to calculate daily compensation or eligibility, though such usage is not universal and should be defined within the organization.

Example: if a company defines the first weekday of the month as the base day, monthly performance is evaluated relative to that day, and any deviations are reported as a percentage change from the peruspäivän level.

Related terms include baseline, anchor date, and reference day.