The first date as a Date object or timestamp (number)
The second date as a Date object or timestamp (number)
The difference in calendar days (negative if dateLeft is before dateRight), or NaN if any input is invalid
// Basic calendar day difference
const result = diffDays(new Date(2024, 5, 15), new Date(2024, 5, 14));
// Returns: 1
// Time components are ignored (same calendar day)
const result = diffDays(new Date(2024, 5, 15, 23, 59), new Date(2024, 5, 15, 0, 0));
// Returns: 0
// Works with timestamps
const timestamp1 = new Date(2024, 5, 20).getTime();
const timestamp2 = new Date(2024, 5, 15).getTime();
const result = diffDays(timestamp1, timestamp2);
// Returns: 5
// Negative result when first date is earlier
const result = diffDays(new Date(2024, 5, 10), new Date(2024, 5, 15));
// Returns: -5
// Invalid inputs return NaN
const result = diffDays(new Date("invalid"), new Date(2024, 5, 15));
// Returns: NaN
Calculate the difference in calendar days between two dates.
This function calculates the number of full calendar days between two dates by comparing them at midnight. Time components are ignored to ensure accurate calendar day counting.