Generating a date variable that is 9 moths after an existing date variable in my dataset

[Noha Hassan]

Hello,

I have a dataset that includes a certain date variable and I want to create another date variable in way that the new variable will be 9 months after the original variable. Example:

var newvar
01aug2021 01may2022
12dec2021 12sep2022

Any idea which commands can I use to apply this?

Thanks in advance.

[Andrew Musau]

It depends how you define months. If it is 30 days, you add 30*9 to the original variable as Stata dates are in units of days. Your example corresponds to wanted2 below, and the implied duration is 8 months.

set obs 2
g var= cond(_n==1, td(01aug2021), td(12dec2021))
*MONTH= 30 DAYS
g wanted1= var+ (30*8)
*MONTH AS DEFINED IN #1
g wanted2= mdy(cond(month(var)<=4, month(var)+8, month(var)+8-12), day(var), cond(month(var)<=4, year(var), year(var)+1))
format var wanted? %td
l

Res.:

. l

     +-----------------------------------+
     |       var     wanted1     wanted2 |
     |-----------------------------------|
  1. | 01aug2021   29mar2022   01apr2022 |
  2. | 12dec2021   09aug2022   12aug2022 |
     +-----------------------------------+

[Nick Cox]

Note that a different definition -- same day of the month but 9 months later -- is problem-free only if this month and that 9 months later have the same number of days, ,
which is true only for May,July and December.

. clear

. set obs 12
Number of observations (_N) was 0, now 12.

. gen sandbox = mdy(_n + 1, 1, 2021) -1
(1 missing value generated)

. replace sandbox = mdy(1, 1, 2022) - 1 in L
(1 real change made)

.
. gen checking = cond(_n <= 3, mdy(_n + 9, 1, 2021) - 1, mdy(_n - 3, 1, 2022) - 1)

. format checking sandbox %td

. list, sep(3)

     +-----------------------+
     |   sandbox   checking  |
     |-----------------------|
  1. | 31jan2021   30sep2021 |
  2. | 28feb2021   31oct2021 |
  3. | 31mar2021   30nov2021 |
     |-----------------------|
  4. | 30apr2021   31dec2021 |
  5. | 31may2021   31jan2022 |
  6. | 30jun2021   28feb2022 |
     |-----------------------|
  7. | 31jul2021   31mar2022 |
  8. | 31aug2021   30apr2022 |
  9. | 30sep2021   31may2022 |
     |-----------------------|
 10. | 31oct2021   30jun2022 |
 11. | 30nov2021   31jul2022 |
 12. | 31dec2021   31aug2022 |
     +-----------------------+

[Noha Hassan]

Thank you so much, Andrew.
Thank you so much, Nick

That was really helpful!