I may have gone a long way around this but is there a way to shorted this statement?

=IF('Paid Until'!D21=1, "1", IF('Paid Until'!D21=2, "1 2", IF('Paid Until'!D21=3, "1 2 3",IF('Paid Until'!D21=4, "1 2 3 4",IF('Paid Until'!D21=5, "1 2 3 4 5",IF('Paid Until'!D21=6, "1 2 3 4 5 6",IF('Paid Until'!D21=7, "1 2 3 4 5 6 7",IF('Paid Until'!D21=8, "1 2 3 4 5 6 7 8",IF('Paid Until'!D21=9, "1 2 3 4 5 6 7 8 9",IF('Paid Until'!D21=10, "1 2 3 4 5 6 7 8 9 10",IF('Paid Until'!D21=11, "1 2 3 4 5 6 7 8 9 10 11", IF('Paid Until'!D21=12, "1 2 3 4 5 6 7 8 9 10 11 12",IF('Paid Until'!D21=13, "1 2 3 4 5 6 7 8 9 10 11 12 13",IF('Paid Until'!D21=14, "1 2 3 4 5 6 7 8 9 10 11 12 13 14",IF('Paid Until'!D21=15, "1 2 3 4 5 6 7 8 9 10 11 12 13 14 15" )))))))))))))))


this works fine but would be better if there is a shorter way to do it lol. thanks