End date by monthly duration given daily start date

[Marc Kaulisch]

I am struggling with a good way to calculate an end date based on a start date (daily date) and a duration on a monthly base. It is a similar question as posted here: https://www.statalist.org/forums/for...-in-my-dataset But I struggle to get a good solution out of this:

My data looks like

* Example generated by -dataex-. For more info, type help dataex
clear
input str9 start_date_s int duration
"01-Oct-13" 66
"01-Nov-13" 66
"30-Oct-15" 62
"01-Apr-15" 48
"01-Sep-15" 57
"01-Dec-15" 66
"01-Nov-15" 60
"01-Nov-15" 48
"01-Feb-16" 41
"01-Jan-16" 45
"01-Jan-16" 48
"01-Jan-16" 36
"01-Dec-16" 54
"01-Jun-17" 69
"01-Jul-17" 72
"18-Apr-17" 24
"01-Jan-18" 70
"01-Apr-18" 66
"01-Apr-18" 24
"01-Jan-18" 69
end

My current way is as follows:

**# start to end date
gen start_date_d = date(start_date_s,"DM20Y")
format start_date_d %td
la var start_date_d "Start Date"

gen start_date_m = mofd(start_date_d)
format start_date_m %tm
la var start_date_m "Start Date (month)"

gen end_date_m = start_date_m + duration - 1
format end_date_m %tm
la var end_date_m "End Date Month"

**## enddate based on last day of end month
gen end_date_d = lastdayofmonth(dofm(end_date_m))
format end_date_d %td
la var end_date_d "End Date"

The last two paragraphs taking function lastdayofmonth(). It only works in cases when start date is the first day of a month but in cases like 30-Oct-15 or 18-Apr-17 it does not work because they should end 29th or 17th respectively.
So basically I would need to calculate the real number of days by months affected.
Is there another function I do not see or does someone has code automatically doing it and is willing to share?

[Marc Kaulisch]

I think I have found a solution:

gen end_date_m = start_date_m + duration - 1 if day(start_date_d) == 1
replace end_date_m = start_date_m + duration if day(start_date_d) ~= 1
format end_date_m %tm
la var end_date_m "End Date Month"

**## enddate based on last day of end month
gen end_date_d = lastdayofmonth(dofm(end_date_m))  if day(start_date_d) == 1
replace end_date_d = (dofm(end_date_m)+ day(start_date_d) - 2) if day(start_date_d) ~= 1
format end_date_d %td
la var end_date_d "End Date"

[Nick Cox]

You said your problem was similar to that in the linked thread, but it doesn't appear to be identical. I start with the idea that you have a calculation in daily dates

end date = start date + duration

so how does month, or the end of the month, come into this?

I am missing something, I fear, but if you go from one daily date to another, the months they are in are immediately evident and accessible.

[Marc Kaulisch]

so how does month, or the end of the month, come into this?

The duration is measured in months. But the end date should be printed as daily date.