Extracting years

[Raphael George]

Hello I wanted to extract the year and month as separate variables from these dates. The year should be either 2007 or 2008. I suppose the last 2 digits denote the year. How do I separate it to say year==2007 or 2008.

* Example generated by -dataex-. To install: ssc install dataex
clear
input long B2_q2i
140608
130608
120608
130608
130608
110608
130608
110608
140608
140608
190608
170608
 90708
140608
180608
150608
200608
160608
180608
170608
 40608
 50608
 80608
 80608
 50608
 80608
 50608
 60608
 40608
 60608
130608
140608
120608
160608
150608
160608
160608
150608
140608
130608
 70508
 40508
 60508
 60508
 80508
 50508
 80508
 90508
 70508
 50508
270408
220408
240408
240408
250408
230408
230408
240408
240408
230408
310508
310508
290508
300508
310508
260508
310508
260508
310508
310508
140408
110408
110408
110408
120408
120408
130408
120408
140408
130408
200408
230408
230408
140408
220408
220408
230408
250408
200408
240408
160608
 70608
140608
140608
150608
150608
160608
 70608
170608
170608
end

------------------ copy up to and including the previous line ------------------

Listed 100 out of 125578 observations
Use the count() option to list more

[Nick Cox]

This all yields to some fooling around with standard functions.

There is no road to understanding Stata dates and times that doesn't lead through

help datetime

where some basic functions are discussed.

In addition, the functions highlighted at https://journals.sagepub.com/doi/pdf...867X1101100308 are useful too and allow a direct approach. Evidently in this data example the year is 2000 plus the modulus on dividing the variable shown by 100, and so forth.

But watch out: in your full dataset you may have some years of the form 19xx, in which case you need something more like

gen twodigityear = mod(B2_q2i, 100)
gen YEAR = cond(twodigityear > 23, 1900 + twodigityear, 2000 + twodigityear)

* Example generated by -dataex-. To install: ssc install dataex
clear
input long B2_q2i
140608
130608
120608
130608
130608
110608
130608
110608
140608
140608
190608
170608
 90708
140608
180608
150608
200608
160608
180608
170608
 40608
 50608
 80608
 80608
 50608
 80608
 50608
 60608
 40608
 60608
130608
140608
120608
160608
150608
160608
160608
150608
140608
130608
 70508
 40508
 60508
 60508
 80508
 50508
 80508
 90508
 70508
 50508
270408
220408
240408
240408
250408
230408
230408
240408
240408
230408
310508
310508
290508
300508
310508
260508
310508
260508
310508
310508
140408
110408
110408
110408
120408
120408
130408
120408
140408
130408
200408
230408
230408
140408
220408
220408
230408
250408
200408
240408
160608
 70608
140608
140608
150608
150608
160608
 70608
170608
170608
end

duplicates drop 

* daily dates 
gen ddate = daily(strofreal(B2_q2i, "%06.0f"), "DM20Y")
format ddate %td 

* monthly dates 
gen mdate = mofd(ddate)
format mdate %tm 

* year and month 
gen year = year(ddate)
gen month = month(ddate)

* a direct route
gen YEAR = 2000 + mod(B2_q2i, 100)

* check it gives the same answer 
assert year == YEAR 

* also direct 
gen MONTH = floor(mod(B2_q2i, 1000)/100)
assert month == MONTH 

sort ddate

list , sepby(month) 

    +-----------------------------------------------------------+
     | B2_q2i       ddate    mdate   year   month   YEAR   MONTH |
     |-----------------------------------------------------------|
  1. | 110408   11apr2008   2008m4   2008       4   2008       4 |
  2. | 120408   12apr2008   2008m4   2008       4   2008       4 |
  3. | 130408   13apr2008   2008m4   2008       4   2008       4 |
  4. | 140408   14apr2008   2008m4   2008       4   2008       4 |
  5. | 200408   20apr2008   2008m4   2008       4   2008       4 |
  6. | 220408   22apr2008   2008m4   2008       4   2008       4 |
  7. | 230408   23apr2008   2008m4   2008       4   2008       4 |
  8. | 240408   24apr2008   2008m4   2008       4   2008       4 |
  9. | 250408   25apr2008   2008m4   2008       4   2008       4 |
 10. | 270408   27apr2008   2008m4   2008       4   2008       4 |
     |-----------------------------------------------------------|
 11. |  40508   04may2008   2008m5   2008       5   2008       5 |
 12. |  50508   05may2008   2008m5   2008       5   2008       5 |
 13. |  60508   06may2008   2008m5   2008       5   2008       5 |
 14. |  70508   07may2008   2008m5   2008       5   2008       5 |
 15. |  80508   08may2008   2008m5   2008       5   2008       5 |
 16. |  90508   09may2008   2008m5   2008       5   2008       5 |
 17. | 260508   26may2008   2008m5   2008       5   2008       5 |
 18. | 290508   29may2008   2008m5   2008       5   2008       5 |
 19. | 300508   30may2008   2008m5   2008       5   2008       5 |
 20. | 310508   31may2008   2008m5   2008       5   2008       5 |
     |-----------------------------------------------------------|
 21. |  40608   04jun2008   2008m6   2008       6   2008       6 |
 22. |  50608   05jun2008   2008m6   2008       6   2008       6 |
 23. |  60608   06jun2008   2008m6   2008       6   2008       6 |
 24. |  70608   07jun2008   2008m6   2008       6   2008       6 |
 25. |  80608   08jun2008   2008m6   2008       6   2008       6 |
 26. | 110608   11jun2008   2008m6   2008       6   2008       6 |
 27. | 120608   12jun2008   2008m6   2008       6   2008       6 |
 28. | 130608   13jun2008   2008m6   2008       6   2008       6 |
 29. | 140608   14jun2008   2008m6   2008       6   2008       6 |
 30. | 150608   15jun2008   2008m6   2008       6   2008       6 |
 31. | 160608   16jun2008   2008m6   2008       6   2008       6 |
 32. | 170608   17jun2008   2008m6   2008       6   2008       6 |
 33. | 180608   18jun2008   2008m6   2008       6   2008       6 |
 34. | 190608   19jun2008   2008m6   2008       6   2008       6 |
 35. | 200608   20jun2008   2008m6   2008       6   2008       6 |
     |-----------------------------------------------------------|
 36. |  90708   09jul2008   2008m7   2008       7   2008       7 |
     +-----------------------------------------------------------+