Type Alias RelativeTimeFormatUnitSingular

RelativeTimeFormatUnitSingular:
    | "year"
    | "quarter"
    | "month"
    | "week"
    | "day"
    | "hour"
    | "minute"
    | "second"

Value of the unit property in objects returned by Intl.RelativeTimeFormat.prototype.formatToParts(). formatToParts and format methods accept either singular or plural unit names as input, but formatToParts only outputs singular (e.g. "day") not plural (e.g. "days").

MDN.