Calculate the days, workdays, weeks, and months between two dates. Supports date addition/subtraction and countdown timers.
Days calculation: Convert dates to millisecond timestamps, subtract, and divide by 86,400,000 (milliseconds per day)
Workday calculation: Excludes Saturdays and Sundays (does not account for public holidays)
Leap year rule: Divisible by 4 but not by 100, or divisible by 400
Date addition: Automatically handles month boundaries, year transitions, and leap years