There are two major versions of Drupal available today 7 and 8. Drupal 7 was released on January 5, 2011 and will be supported until November 2021. Drupal 8 was released on November 19, 2015 and will be supported until November 2021. Meanwhile on 2020 Drupal 9 is announced to be released. Drupal 9 will be released one year before both 7 an 8 versions will reach there end-of-life. That time will be sufficient to upgrade to Drupal 9. And moving from Drupal 8 to 9 is promised to be as easy as keep you current Drupal version up to date. Drupal 7 users might safely continue to run there systems with regular PHP updates till 2021, then they have to upgrade to Drupal 9. Alternatively version 7 users might upgrade it to version 8 before 2020 and then easily update to Drupal 9 on 2021.