print('This program will tell you the year you turn 100 years old.')

print('\nWhat is your name?')

namecheck = 0
while namecheck == 0:
  name = input('Name: ')
  if name.isalpha() and len(name) < 30:
    while True:
      name = name.title()
      yesno = input('Your name is {0}. Is this correct? (Yes/No) '.format(name))
      if yesno.lower() == 'yes':
        namecheck = 1
        break
      elif yesno.lower() == 'no':
        break
      else:
        print('Type "Yes" or "No".')
  if not name.isalpha():
    print('Please only use letters from the Roman alphabet.')
  if len(name) > 30:
    print('Please use less than 30 characters.')

import datetime
now = datetime.datetime.now() # current date and time

print('\nHow old are you?')

agecheck = 0
birthday = 1
while agecheck == 0:
  age = input('Age: ')
  if age.isdigit():
    if birthday == 0: # if computed birthday was wrong, user inputs it
      print('Which year were you born in?')
      while True:
        birthday = input('Year: ')
        if len(birthday) != 4:
            print('Please enter a valid date.')
        if int(age)+int(birthday) == now.year or int(age)+int(birthday)+1 == now.year:
            break
        else:
          print('Your entered age is {0}. This does not make sense with the year you have entered.'.format(age))
    else:
      birthday = now.year-int(age)
    if int(age) > 1: # differentiate between "year old" and "years old"
      year_s = 's'
    else:
      year_s = ''
    while True:
      yesno = input('You are {0} year{1} old and were born in {2}. Is this correct? (Yes/No) '.format(age,year_s,str(birthday)))
      if yesno.lower() == 'yes':
        agecheck = 1
        break
      elif yesno.lower() == 'no':
        print('Please re-enter your age and birthday.')
        birthday = 0
        break
      else:
        print('Type "Yes" or "No".')
  else:
    print('Please only use Roman numerals.')

if int(age) < 100:
    turn_100 = int(birthday)+100
    years_until_100 = turn_100-now.year
    print('\n{0}, you are {1} year{2} old.\nIn {3} you will turn 100 years old.\nThis is {4} years away.'.format(name,age,year_s,str(turn_100),str(years_until_100)))
elif int(age) == 100 and now.year-int(birthday) == 100:
    print('{0}, you turned 100 years old this year! Congratulations!'.format(name))
else:
    turn_100 = int(birthday)+100
    years_until_100 = (turn_100-now.year)*-1
    if years_until_100 > 1:
        year_s = 's'
    else:
        year_s = ''
    print('\n{0}, you have already turned 100 years old. This was in {1}, {2} year{3} ago.'.format(name,turn_100,years_until_100,year_s))