import datetime
import re

def sane_year(yearAsString, lookahead=20):
    """Sanity-check years entered as less than four digits.
    
    If users want to enter years between 0 A.D. and 99 A.D., they need to
    enter it as a four-digit year (0000-0099). Dates entered as less than
    four digits take the current century, unless that places them past the
    current year by (lookahead) years or more, in which case they take the
    previous century."""
    yearAsInt = int(yearAsString)
    if yearAsInt < 100 and len(yearAsString) < 4:
        currentYear = datetime.date.today().year
        currentCentury, rem = divmod(currentYear, 100)
        yearAsInt += (currentCentury * 100)
        if yearAsInt >= (currentYear + lookahead):
            yearAsInt -= 100
    return yearAsInt

def parse_date(dateAsString):
    atoms = re.split(r"\D", dateAsString)
    if len(atoms) != 3:
        raise ValueError("Date values must be in the form: mm dd yy.")
    atoms = sane_year(atoms[2]), int(atoms[0]), int(atoms[1])
    return atoms