Leap day on a leap year
Input
validate_date('2020-02-29')
Output
True
2020 is a leap year and 02-29 is a valid date.
Full lesson preview
Validate a date string in YYYY-MM-DD format
Check whether a string is a valid date in strict YYYY-MM-DD format, including leap years.
Python practice14 minError Handling & Edge CasesIntermediateLast updated March 20, 2026
Problem statement
Create a function validate_date(date_str) that returns True if date_str is a valid date in the exact YYYY-MM-DD format, otherwise returns False. The function should:
- Require a string input. Non-string inputs return False.
- Enforce exactly four digits for the year, two digits for the month, and two digits for the day, separated by single hyphens.
- Validate month is 01-12 and day is valid for that month, taking leap years into account.
- Reject invalid numeric values (e.g., month 00 or 13, day 00 or out of range).
- Reject extra characters, missing parts, or different separators.
Write robust defensive checks and handle malformed inputs without raising exceptions.
Task
Write a function that verifies input is a string formatted as YYYY-MM-DD and represents a real calendar date.
Examples
Input format
A single argument: date_str (string) to validate.
Output format
Return True if valid YYYY-MM-DD date, otherwise False.
Constraints
Do not use uncontrolled exception propagation. Use standard library if helpful. Year must be exactly 4 digits, month/day exactly 2 digits.
Samples
Sample 1
Input
validate_date('2019-02-28')
Output
True
2019-02-28 is a valid non-leap-year date.