Calculating the difference between times (not dates) for data read as a string

[Bob Green]

Hello,

I am wanting to calculate the time in minutes between pairs of selected variables. The first pair of variables (starttime endtime) were read in as strings. I tried to destring these variables but received an error stating they contained non-numeric characters:

. destring starttime, gen (start)
starttime contains nonnumeric characters; no generate


For some reason, the other set of variables I entered were accepted as numeric characters. When I deducted Break12 - Break11 I obtained two values: 5400000 & 7260000

My two questions are:

1) what is required to substract the difference between starttime and endtime
2) what is the format of the output obtained by deducting Break12 - Break11? Looking at the raw data it should be 90 minutes: 20:12 - 18:42 (which is how the data was entered)

Any assistance is appreciated.

Bob

* Example generated by -dataex-. To install: ssc install dataex
clear
input int id str5(starttime endtime) double(Break11 Break12 Break21 Break22 Break31 Break32)
 3 "11:52" " 3:15"              .              .              .                   .              .              .
 4 " 5:13" "10:37" -1893388680000 -1893383280000 -1893378060000 -1893376979999.9998              .              .
 5 " 8:38" " 8:57"              .              .              .                   .              .              .
 7 " "     "12:10"              .              .              .                   .              .              .
14 " 2:46" " 8:31"              .              .              .                   .              .              .
15 " 1:45" "12:16" -1893449280000 -1893442020000 -1.8934347e+12 -1893433380000.0002 -1.8934293e+12 -1893412980000
end
format %tchh:MM Break11
format %tchh:MM Break12
format %tchh:MM Break21
format %tchh:MM Break22
format %tchh:MM Break31
format %tchh:MM Break32

[Nick Cox]

You should almost never want to destring dates or times. The only likely exception might be string years such as "2015" "2016". If you destring "23:00" and "22:59" (which is possible by ignoring the colons) then arithmetic will give you a nonsense difference of 2300 - 2259 = 41, not 1.

But you have a good answers, like 5400000.

You just need to see why you get those. As you're telling us you have date-times with display formats %tc and the units are milliseconds.

If you want to use results in minutes you just divide by 60000.

All documented e.g. at help datetime

[Bob Green]

Nick,

Many thanks for clarifying how to calculate minutes for the variables with the double format. I just saw another post about using format %tc. I'll see if this works on the str5 variables (starttime endtime) .

[Bob Green]

For the conversion from string to a time variable it looks like what I needed is: gen double starttime2 = clock(starttime, "hm")