TheAlgorithms/Python · error · ValueError

Date separator must be '-' or '/'

Error message

Date separator must be '-' or '/'

What it means

Raised by zeller(date_input) in maths/zellers_congruence.py when the character at position 2 (the separator between month and day) is not '-' or '/'. The parser is strictly positional: chars 2 and 5 must be separators, digits are taken from fixed offsets. This is the first-separator check, hit by inputs like '01^31-2010'.

Source

Thrown at maths/zellers_congruence.py:100

        "6": "Saturday",
    }

    convert_datetime_days = {0: 1, 1: 2, 2: 3, 3: 4, 4: 5, 5: 6, 6: 0}

    # Validate
    if not 0 < len(date_input) < 11:
        raise ValueError("Must be 10 characters long")

    # Get month
    m: int = int(date_input[0] + date_input[1])
    # Validate
    if not 0 < m < 13:
        raise ValueError("Month must be between 1 - 12")

    sep_1: str = date_input[2]
    # Validate
    if sep_1 not in ["-", "/"]:
        raise ValueError("Date separator must be '-' or '/'")

    # Get day
    d: int = int(date_input[3] + date_input[4])
    # Validate
    if not 0 < d < 32:
        raise ValueError("Date must be between 1 - 31")

    # Get second separator
    sep_2: str = date_input[5]
    # Validate
    if sep_2 not in ["-", "/"]:
        raise ValueError("Date separator must be '-' or '/'")

    # Get year
    y: int = int(date_input[6] + date_input[7] + date_input[8] + date_input[9])
    # Arbitrary year range
    if not 45 < y < 8500:
        raise ValueError(

View on GitHub (pinned to f5988cc097)

Solutions

1. Reformat the date with '-' separators before calling: s.replace('.', '-').replace('/', '-') after verifying overall shape.
2. Best: rebuild the string from a parsed date via strftime('%m-%d-%Y') so separators are guaranteed correct.
3. Pre-validate with a regex such as ^\d{2}[-/]\d{2}[-/]\d{4}$ and reject mismatches early.

Example fix

# before
zeller('01.31.2010')  # '.' separator -> ValueError

# after
zeller('01.31.2010'.replace('.', '-'))  # '01-31-2010'

import re

def has_valid_separators(s: str) -> bool:
    return len(s) == 10 and s[2] in '-/' and s[5] in '-/'

normalized = re.sub(r'[.\s]+', '-', s)

try:
    day = zeller(s)
except ValueError as e:
    if 'separator' in str(e):
        day = zeller(re.sub(r'[. ]', '-', s))
    else:
        raise

When it happens

Trigger: Calling zeller('01.31.2010'), zeller('01 31 2010') or zeller('01^31-2010'); dates produced with '.', ' ' or other separators; strings where a leading character shifted positions.

Common situations: Locale-specific separators (dots in European dates, dots/spaces in ISO-adjacent formats); copy-pasted dates with non-breaking spaces; downstream systems reformatting the string.

Related errors

• Must be 10 characters long
• Month must be between 1 - 12
• Date must be between 1 - 31
• Power cannot be negative in any electrical/electronics syste
• One and only one argument must be 0

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/96d84dedd8ab126e. Report an issue: GitHub.