How to substract 1 year from a ddmmyyyy variable

[Alice Yang]

Dear statalist,

I have some dates, such as 05sep2014, in %td format, and I would like to create the same date in the previous year, so in this case, 05sep2013, how to do this? Thanks.

[Ken Chui]

clear
input x1
19971
end
format x1 %td

gen x2 = mdy(month(x1), day(x1), year(x1)-1)
format x2 %td

list

Results:

     +-----------------------+
     |        x1          x2 |
     |-----------------------|
  1. | 05sep2014   05sep2013 |
     +-----------------------+

[William Lisowski]

There is no "same date in the previous year" when the date is a leap year day. You have to make a choice.

clear
input x1
19971
21940
21974
21975
21985
end
format x1 %td

gen x2 = mdy(month(x1), day(x1), year(x1)-1)
format x2 %td

gen x3f = mdy(month(x1), day(x1), year(x1)-1)
replace x3f = mdy(month(x1), day(x1-1), year(x1)-1) if missing(x3f)

gen x3m = mdy(month(x1), day(x1), year(x1)-1)
replace x3m = mdy(month(x1)+1,1, year(x1)-1) if missing(x3m)

format x3f x3m %td

list

. list

     +-----------------------------------------------+
     |        x1          x2         x3f         x3m |
     |-----------------------------------------------|
  1. | 05sep2014   05sep2013   05sep2013   05sep2013 |
  2. | 26jan2020   26jan2019   26jan2019   26jan2019 |
  3. | 29feb2020           .   28feb2019   01mar2019 |
  4. | 01mar2020   01mar2019   01mar2019   01mar2019 |
  5. | 11mar2020   11mar2019   11mar2019   11mar2019 |
     +-----------------------------------------------+

[Alice Yang]

Hi Ken and William,

Thank you both for the kind help!