When you need to find out how much time separates two dates, the Day.js diff method is your primary tool. This method allows you to calculate the difference between two dates in various units, such as days, months, or years. The result of diff can be either positive or negative, depending on the order of the dates: if the first date is later than the second, the result is positive; if the first date is earlier, the result is negative. This behavior makes it easy to determine not only the magnitude of the difference but also which date comes first.

You can specify the unit of measurement by passing a string such as "day", "month", or "year" as the second argument to diff. This flexibility lets you choose the level of granularity that fits your needs. For example, calculating the difference in days gives you the total number of days between two dates, while using months or years can help you track longer periods.

Edge cases can arise when dates fall at the end or beginning of months, or during leap years. Day.js handles these situations according to its internal logic, which means the result will always be an integer that reflects the number of complete units between the two dates. If you need more precise differences, such as including partial units, you can pass an additional argument to diff, but by default, it rounds down to the nearest whole unit.

Understanding the sign and value of the result is crucial for tasks like checking deadlines, measuring durations, or building features that depend on time calculations.