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    Function setMonth

    • Set the month of the given date.

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

      Parameters

      • date: number | Date

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

      • month: number

        The month to set (0-indexed: 0 = January, 11 = December)

      Returns Date

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

      // Set month to June
      const result = setMonth(new Date(2025, 0, 15), 5);
      // Returns: 2025-06-15

      // Set month to January
      const result2 = setMonth(new Date(2025, 5, 15), 0);
      // Returns: 2025-01-15

      // Day overflow adjustment (Jan 31 → Feb 28)
      const result3 = setMonth(new Date(2025, 0, 31), 1);
      // Returns: 2025-02-28 (non-leap year)

      // Fractional month is truncated
      const result4 = setMonth(new Date(2025, 0, 15), 5.9);
      // Returns: 2025-06-15

      // Invalid date returns Invalid Date
      const result5 = setMonth(new Date("invalid"), 5);
      // Returns: Invalid Date
      • Validates arguments before conversion (consistent with library patterns)
      • Accepts both Date objects and numeric timestamps
      • Fractions are truncated using Math.trunc (5.9 → 5, -1.9 → -1)
      • Returns Invalid Date for: Invalid Date, NaN, Infinity, -Infinity
      • Preserves year, day, and time components (hours, minutes, seconds, milliseconds)
      • Special handling: When the original day doesn't exist in the target month (e.g., Jan 31 → Feb), adjusts to the last valid day of the month (Feb 28 or 29)
      • Always returns a new Date instance (does not mutate input)

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