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    Function addMilliseconds

    • Add the specified number of milliseconds to the given date.

      This function validates arguments before processing and returns a new Date instance with the specified number of milliseconds added. Fractional milliseconds are truncated toward zero.

      Parameters

      • date: number | Date

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

      • amount: number

        The number of milliseconds to add (can be negative to subtract)

      Returns Date

      A new Date object with the milliseconds added, or Invalid Date if any input is invalid

      // Add positive milliseconds
      const result = addMilliseconds(new Date(2020, 0, 1, 12, 0, 0, 0), 500);
      // Returns: 2020-01-01T12:00:00.500

      // Subtract milliseconds (negative amount)
      const result = addMilliseconds(new Date(2020, 0, 1, 12, 0, 0, 500), -300);
      // Returns: 2020-01-01T12:00:00.200

      // Fractional amounts are truncated
      const result = addMilliseconds(new Date(2020, 0, 1, 12, 0, 0, 0), 1.9);
      // Returns: 2020-01-01T12:00:00.001 (1.9 truncated to 1)

      // Crossing second boundary
      const result = addMilliseconds(new Date(2020, 0, 1, 12, 0, 0, 999), 1);
      // Returns: 2020-01-01T12:00:01.000

      // Invalid inputs return Invalid Date
      const result = addMilliseconds(new Date("invalid"), 500);
      // Returns: Invalid Date
      • Validates arguments before conversion (consistent with library patterns)
      • Accepts both Date objects and numeric timestamps
      • Fractions are truncated using Math.trunc (1.9 → 1, -1.9 → -1)
      • Returns Invalid Date for: Invalid Date, NaN, Infinity, -Infinity
      • Always returns a new Date instance (does not mutate input)

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