The earth is alive and well, and still it will be here, after the 21st. If the world "ends" then on what will Jesus reign on for 1000 years? The new heavens and earth will not become into being until after the 1000th year of Christ's rule.
The only thing that will "end", at a date still yet unknown, is man's rule and satan's dominion/rule of the affairs of earth, when Christ Raptures his own and begins the 7years of God's wrath upon all who dis-obey Gods good news, the Gospel by refusing the free and open gift of eternal salvation and sabbath rest found only in Jesus.
Better to see him and make him as King now, than to lift up ones head, and see him as King, and Judge later, at the great white throne.