Generate new variable with the date from the previous year (Problem with leap years)

[Nick Bertel]

Dear all,

I want to create a new variable that has the date of the previous year in it:
This is my code so far:

gen leap = mod(year(FPEDATS), 4) == 0 & (mod(year(FPEDATS), 100) != 0 | mod(year(FPEDATS), 400) == 0)
gen FPEDATSn_1 = FPEDATS - (365 + leap)
format %td FPEDATSn_1

but it doesn't work if the variable FPEDATS is in January and in a leap year as it then substracts one to many (e.g. out of 31jan2016 becomes 30jan2015)
Also if the desired previous year FPEDATSn_1 is a leap year then out of e.g. 31jan2017 becomes 01feb2016

Does someone know how to do it right?

this is part of my data:

* Example generated by -dataex-. For more info, type help dataex
clear
input long FPEDATS
20850
22311
20850
22311
22339
22704
22311
22311
22311
22311
22311
22311
22311
22704
22311
22311
22311
22339
22704
12084
13545
15006
16467
16467
12084
13545
16467
17928
19389
22311
15006
13545
15006
16467
17928
19389
20850
22311
12084
13208
15034
15006
22311
15006
16467
17928
19389
20850
12084
13545
15006
16467
17928
19389
13545
16467
17928
19389
22311
13545
18686
12842
13573
15034
15399
15764
16130
16495
16860
17225
17591
17956
18321
18686
19052
19417
19782
20878
21974
22339
22704
11747
12112
12842
13208
13573
14303
14669
15034
15399
15764
16130
16495
16860
17225
17591
17956
18321
18686
19052
end
format %td FPEDATS

Thanks!

[Nick Cox]

See if https://www.statalist.org/forums/for...ar-in-the-date solves your problem.

[Nick Bertel]

thank you! but that still leaves me with one problem that i maybe didnt specify enough: If we have the date 28feb2009 then i need it for the previous year to be the last day of the month aswell which would be 29feb2008. How would i go about that? Identiffy that the wanted year is leap and that add 1 if the month is february?

[Nick Cox]

OK.

The last day of the month is the day before the first day of the next month, always. No need to worry about leap years.

* Example generated by -dataex-. For more info, type help dataex
clear
input long FPEDATS
11747
12084
12112
12842
13208
13545
13573
14303
14669
15006
15034
15399
15764
16130
16467
16495
16860
17225
17591
17928
17956
18321
18686
19052
19389
19417
19782
20850
20878
21974
22311
22339
22704
end
format %td FPEDATS

gen wanted = dofm(ym(year(FPEDATS) - 1, month(FPEDATS) + 1)) - 1

format %td wanted

list

     +-----------------------+
     |   FPEDATS      wanted |
     |-----------------------|
  1. | 29feb1992   28feb1991 |
  2. | 31jan1993   31jan1992 |
  3. | 28feb1993   29feb1992 |
  4. | 28feb1995   28feb1994 |
  5. | 29feb1996   28feb1995 |
     |-----------------------|
  6. | 31jan1997   31jan1996 |
  7. | 28feb1997   29feb1996 |
  8. | 28feb1999   28feb1998 |
  9. | 29feb2000   28feb1999 |
 10. | 31jan2001   31jan2000 |
     |-----------------------|
 11. | 28feb2001   29feb2000 |
 12. | 28feb2002   28feb2001 |
 13. | 28feb2003   28feb2002 |
 14. | 29feb2004   28feb2003 |
 15. | 31jan2005   31jan2004 |
     |-----------------------|
 16. | 28feb2005   29feb2004 |
 17. | 28feb2006   28feb2005 |
 18. | 28feb2007   28feb2006 |
 19. | 29feb2008   28feb2007 |
 20. | 31jan2009   31jan2008 |
     |-----------------------|
 21. | 28feb2009   29feb2008 |
 22. | 28feb2010   28feb2009 |
 23. | 28feb2011   28feb2010 |
 24. | 29feb2012   28feb2011 |
 25. | 31jan2013   31jan2012 |
     |-----------------------|
 26. | 28feb2013   29feb2012 |
 27. | 28feb2014   28feb2013 |
 28. | 31jan2017   31jan2016 |
 29. | 28feb2017   29feb2016 |
 30. | 29feb2020   28feb2019 |
     |-----------------------|
 31. | 31jan2021   31jan2020 |
 32. | 28feb2021   29feb2020 |
 33. | 28feb2022   28feb2021 |
     +-----------------------+

See also

SJ-19-3 dm0100 . . . . . . . . . . Speaking Stata: The last day of the month
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. J. Cox
Q3/19 SJ 19(3):719--728 (no commands)
discusses three related problems about getting the last day
of the month in a new variable

Stata now has a lastdayofmonth() function introduced fairly recently, but this solution works some versions back.

[Nick Bertel]

Thank you although it looks like that doesnt work if the date is on the 31 of December unfortunately

[Nick Cox]

You're right. Sorry about that. Let's try again:

* Example generated by -dataex-. For more info, type help dataex
clear
input long FPEDATS
19417
19782
20850
20878
21974
22311
22339
22704
23010
end
format %td FPEDATS

gen wanted = dofm(mofd(FPEDATS) - 11) - 1

format %td wanted 

list 

     +-----------------------+
     |   FPEDATS      wanted |
     |-----------------------|
  1. | 28feb2013   29feb2012 |
  2. | 28feb2014   28feb2013 |
  3. | 31jan2017   31jan2016 |
  4. | 28feb2017   29feb2016 |
  5. | 29feb2020   28feb2019 |
     |-----------------------|
  6. | 31jan2021   31jan2020 |
  7. | 28feb2021   29feb2020 |
  8. | 28feb2022   28feb2021 |
  9. | 31dec2022   31dec2021 |
     +-----------------------+

[Nick Bertel]

Thanks a lot