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    Function startOfMonth

    • Get the start of the month for the given date.

      This function returns a new Date object set to the first day of the month at 00:00:00.000 for the given date. The year and month remain the same while the day is set to 1 and all time components are reset to zero.

      Parameters

      • date: number | Date

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

      Returns Date

      A new Date object set to the first day of the month at 00:00:00.000, or Invalid Date if input is invalid

      // Get start of month from mid-month
      const result = startOfMonth(new Date(2024, 5, 15, 14, 30, 45));
      // Returns: June 1, 2024 00:00:00.000

      // Works with last day of month
      const result2 = startOfMonth(new Date(2024, 5, 30, 23, 59, 59));
      // Returns: June 1, 2024 00:00:00.000

      // Works with timestamps
      const timestamp = Date.now();
      const result3 = startOfMonth(timestamp);
      // Returns: First day of current month at 00:00:00.000

      // Handles leap year February
      const result4 = startOfMonth(new Date(2024, 1, 29));
      // Returns: February 1, 2024 00:00:00.000

      // Invalid inputs return Invalid Date
      const result5 = startOfMonth(NaN);
      // Returns: Invalid Date
      • Validates arguments before processing (consistent with library patterns)
      • Accepts both Date objects and numeric timestamps
      • Returns Invalid Date for: Invalid Date, NaN, Infinity, -Infinity
      • Always returns a new Date instance (does not mutate input)
      • Sets day to 1 and resets hours, minutes, seconds, and milliseconds to 0

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