Function getDay

getDay(date: number | Date): number
Get the day of the month from the given date.

This function validates arguments before processing and returns the day of the month (1-31) 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 day of the month as a number (1-31), or NaN if invalid
Example
```
// Get day from Date object
const result = getDay(new Date(2025, 0, 15));
// Returns: 15

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

// Leap day
const result3 = getDay(new Date(2024, 1, 29));
// Returns: 29

// End of month
const result4 = getDay(new Date(2024, 0, 31));
// Returns: 31

// Invalid date returns NaN
const result5 = getDay(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 day in the local timezone
- Day values range from 1 to 31 depending on the month
- Defined in getDay/index.ts:42