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    Function setDay

    • Set the day of the month of the given date.

      This function validates arguments before processing and returns a new Date instance with the specified day set. Fractional days are truncated toward zero.

      Parameters

      • date: number | Date

        The base date as a Date object or timestamp (number)

      • day: number

        The day to set (1-31, fractions are truncated)

      Returns Date

      A new Date object with the day set, or Invalid Date if any input is invalid

      // Set day to 1st
      const result = setDay(new Date(2025, 0, 15), 1);
      // Returns: 2025-01-01

      // Set day to last day of month
      const result2 = setDay(new Date(2025, 0, 15), 31);
      // Returns: 2025-01-31

      // Day overflow (Jan 32 → Feb 1)
      const result3 = setDay(new Date(2025, 0, 15), 32);
      // Returns: 2025-02-01

      // Fractional day is truncated
      const result4 = setDay(new Date(2025, 0, 15), 15.9);
      // Returns: 2025-01-15

      // Invalid date returns Invalid Date
      const result5 = setDay(new Date("invalid"), 15);
      // Returns: Invalid Date
      • Validates arguments before conversion (consistent with library patterns)
      • Accepts both Date objects and numeric timestamps
      • Fractions are truncated using Math.trunc (15.9 → 15, -15.9 → -15)
      • Returns Invalid Date for: Invalid Date, NaN, Infinity, -Infinity
      • Preserves year, month, and time components (hours, minutes, seconds, milliseconds)
      • Rollover behavior: Day values outside valid range cause date to roll over to adjacent months (e.g., Jan 32 → Feb 1, day 0 → last day of previous month)
      • Always returns a new Date instance (does not mutate input)

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