Function getMonth

getMonth(date: number | Date): number
Get the month of the given date.

This function validates arguments before processing and returns the month (1-12, where January is 1 and December is 12) of the given date. Returns NaN for invalid input.
Parameters
- date: number | Date
  The base date as a Date object or timestamp (number)
Returns number
The month as a number (1-12), or NaN if invalid
Example
```
// Get month from Date object
const result = getMonth(new Date(2024, 5, 15)); // June
// Returns: 6

// Get month from timestamp
const result2 = getMonth(1704067200000); // 2024-01-01
// Returns: 1

// Leap year February
const result3 = getMonth(new Date(2024, 1, 29));
// Returns: 2

// December
const result4 = getMonth(new Date(2024, 11, 25));
// Returns: 12

// Invalid date returns NaN
const result5 = getMonth(new Date("invalid"));
// Returns: NaN
```
Remarks
- Validates arguments before conversion (consistent with library patterns)
- Accepts both Date objects and numeric timestamps
- Returns NaN for: Invalid Date, NaN, Infinity, -Infinity
- Returns the month in the local timezone
- Uses 1-based indexing (1-12) unlike JavaScript's native getMonth() (0-11)
- Defined in getMonth/index.ts:42