The Number of Days Between Two Dates problem asks you to compute the absolute difference in days between two date strings in "YYYY-MM-DD" format. This requires handling leap years correctly and converting dates to a total day count from a common epoch. This coding problem tests math and date arithmetic implementation skills.
Microsoft, Meta, Amazon, and Google ask this to test careful date arithmetic without using library functions. It validates knowledge of leap year rules (divisible by 4, except centuries unless divisible by 400) and the ability to implement a complete date-to-days conversion from scratch. The math and string interview pattern is demonstrated.
Convert both dates to days from epoch + subtract. Convert "YYYY-MM-DD" to total days since a base date (e.g., year 1, day 1). For each date: compute days from full years (accounting for leap years), add days from full months in the current year, add remaining days. Return the absolute difference.
Date 1: "2019-06-29", Date 2: "2019-06-30". Days from epoch for both differ by exactly 1. Result = 1 day.
Date 1: "2000-01-01", Date 2: "2000-03-01" (2000 is a leap year): Jan=31, Feb=29 (leap year), so March 1 is day 60+1=61. Difference from Jan 1 = 60 days.
Date arithmetic from scratch requires correctly implementing the leap year formula: (year % 4 == 0 and year % 100 != 0) or year % 400 == 0. Memorize the days per month array and remember February has 28 or 29 days. Practice computing "days since year 1, January 1" for a given date — this conversion is the core building block for all calendar arithmetic problems. Always test with boundary cases: Feb 28/29, year 2000, year 1900.
