Hi everyone. I need to select the MRIs that are less than 18 months apart.

Thank you very much

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Code:
* Example generated by -dataex-. To install: ssc install dataex clear input str9 id float(nmri date_mri) "10019723" 1 17015 "10019723" 2 21587 "10054335" 1 20746 "10054335" 2 21517 "10054335" 3 21977 "10087618" 1 14741 "10087618" 2 15810 "10087618" 3 16217 "10087618" 4 16579 "10103855" 1 16528 "10103855" 2 16748 "10103855" 3 17242 "10171615" 1 17455 "10171615" 2 21367 "10171615" 3 21755 "10239623" 1 15756 "10239623" 2 15951 "10239623" 3 16245 "10239623" 4 17465 "10239623" 5 17821 "10239623" 6 19354 "10243938" 1 15282 "10243938" 2 15649 "10243938" 3 21584 "10323354" 1 18499 "10323354" 2 18943 "10323354" 3 19306 "10323354" 4 19708 "10323354" 5 19810 "10323354" 6 20105 "10323354" 7 20248 "10323354" 8 20653 "10323354" 9 20973 "10323354" 10 21168 "10323354" 11 21733 "10323354" 12 21930 "10380065" 1 18290 "10380065" 2 19633 "10380065" 3 21025 "10380065" 4 21453 "10380065" 5 21453 "10398574" 1 15401 "10398574" 2 15552 "10398574" 3 15846 "10446691" 1 13724 "10446691" 2 15585 "10446691" 3 17227 "10446691" 4 18863 "10446691" 5 19057 "10451113" 1 18116 "10451113" 2 19731 "10451113" 3 20168 "10456635" 1 20746 "10482466" 1 20464 "10482466" 2 21913 "10490935" 1 13986 "10490935" 2 18936 "10490935" 3 18936 "10541670" 1 17352 "10541670" 2 17353 "10541670" 3 19159 "10541670" 4 20510 "10541670" 5 20972 "10541670" 6 21615 "10541670" 7 22082 "10571702" 1 16813 "10580120" 1 15240 "10580120" 2 15651 "10580120" 3 17367 "10580120" 4 19496 "10580120" 5 20840 "10580120" 6 21284 "10582578" 1 15596 "10582578" 2 15846 "10582578" 3 16273 "10582578" 4 17274 "10582578" 5 18116 "10582578" 6 19096 "10582578" 7 19118 "10600267" 1 15226 "10600267" 2 15637 "10600267" 3 15637 "10638696" 1 16029 "10638696" 2 16420 "10718399" 1 17165 "10718399" 2 17387 "10718399" 3 18943 "10718399" 4 20707 "10718399" 5 21691 "10746760" 1 19697 "10746760" 2 20504 "10840543" 1 15603 "10840543" 2 16013 "10840543" 3 19918 "10840543" 4 20242 "10840543" 5 20482 "10840543" 6 20724 "10840543" 7 21089 "10840543" 8 21560 "10840543" 9 21895 end format %tdDD/NN/CCYY date_mri