#1360

Easy

LeetCode Problem Workspace

Number of Days Between Two Dates

Calculate the exact number of days between two dates using string parsing and arithmetic logic with consistent accuracy.

math string

Problem Statement

Given two dates as strings in the format YYYY-MM-DD, write a program to determine the number of days between them. The solution must correctly handle leap years and varying month lengths, and return a non-negative integer representing the difference in days.

For example, given date1 = "2019-06-29" and date2 = "2019-06-30", the output should be 1. Another example is date1 = "2020-01-15" and date2 = "2019-12-31", which should return 15. Constraints: all dates are valid between 1971 and 2100.

Examples

Example 1

Input: date1 = "2019-06-29", date2 = "2019-06-30"

Output: 1

Example details omitted.

Example 2

Input: date1 = "2020-01-15", date2 = "2019-12-31"

Output: 15

Example details omitted.

Constraints

The given dates are valid dates between the years 1971 and 2100.

Solution Approach

Parse Dates into Year, Month, Day

Split each input string by the '-' character to extract integers representing year, month, and day. This string-to-integer parsing is critical to feed the arithmetic calculations accurately.

Compute Days from a Fixed Reference

Implement a helper function f(date) that counts the total number of days from a base date like 1900-01-01 to the given date. Include leap year logic and month day accumulation. Subtract f(date1) from f(date2) and return the absolute value.

Handle Leap Years and Month Lengths

Use standard rules for leap years: divisible by 4 but not 100 unless divisible by 400. Maintain an array of month lengths for non-leap and leap years. This prevents off-by-one errors in the final day count.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

Time complexity is O(1) because date calculations involve fixed-size arithmetic regardless of input. Space complexity is O(1) as only constant variables and arrays for month lengths are used.

What Interviewers Usually Probe

Expect clear handling of string parsing and date conversion logic.
Look for awareness of leap years and month-specific day counts.
Check for simplicity in computing total days without iterative counting from one date to another.

Common Pitfalls or Variants

Common pitfalls

Ignoring leap years and producing off-by-one errors for February dates.
Swapping date1 and date2 incorrectly or not taking absolute difference.
Assuming all months have 30 or 31 days instead of using correct month lengths.

Follow-up variants

Calculate business days only between two dates, skipping weekends.
Include time in hours and minutes to compute the difference precisely.
Determine the day of the week for each date in addition to the difference.

FAQ

What is the main approach to solve Number of Days Between Two Dates?

Convert each date to total days from a fixed reference using arithmetic and string parsing, then take the absolute difference.

How do leap years affect the calculation?

Leap years add an extra day in February, so the helper function must account for years divisible by 4, excluding 100 unless divisible by 400.

Can I assume months have fixed lengths?

No, each month has a specific number of days, and February varies by leap year; using an array of month lengths avoids miscalculations.

Do I need to worry about negative results?

No, always return the absolute difference between the two day counts regardless of which date comes first.

Is this problem best categorized under Math plus String?

Yes, it requires parsing strings into numerical components and applying arithmetic to compute the day difference accurately.

terminal

Solution

Solution 1: Mathematics

First, we define a function `isLeapYear(year)` to determine whether the given year `year` is a leap year. If it is a leap year, return `true`, otherwise return `false`.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

class Solution:
    def daysBetweenDates(self, date1: str, date2: str) -> int:
        def isLeapYear(year: int) -> bool:
            return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)

        def daysInMonth(year: int, month: int) -> int:
            days = [
                31,
                28 + int(isLeapYear(year)),
                31,
                30,
                31,
                30,
                31,
                31,
                30,
                31,
                30,
                31,
            ]
            return days[month - 1]

        def calcDays(date: str) -> int:
            year, month, day = map(int, date.split("-"))
            days = 0
            for y in range(1971, year):
                days += 365 + int(isLeapYear(y))
            for m in range(1, month):
                days += daysInMonth(year, m)
            days += day
            return days

        return abs(calcDays(date1) - calcDays(date2))

Continue Practicing

#1307 verbal arithmetic puzzle #1447 simplified fractions #1247 minimum swaps to make strings equal