Minus 3 minutes from a datetime

[Chen Samulsion]

Dear Stata users,

I have a data of time variable. Its storage type is string. I want to minus 3 minutes (180000ms) from this time variable. How can I do that? Thank you very much.

* Example generated by -dataex-. To install: ssc install dataex
clear
input strL hms
"00:05:03,000"
"00:05:06,097"
"00:05:09,822"
"00:05:14,312"
"00:05:16,125"
"00:05:18,131"
"00:05:24,190"
"00:05:27,229"
end

[daniel klein]

generate double hms_minus3min = clock(subinstr(hms,",",".",.),"hms")-180000
format hms_minus3min %tcHH:MM:SS.sss

[Chen Samulsion]

Thank you so much daniel klein.
I did the first step, then format newdate %tc, and got result of HH:MM:SS format, because I don't know the use of format %tcHH:MM:SS.sss. Thank you again.

[Hemanshu Kumar]

Hmm, I thought Chen might want the milliseconds separated by a comma, and that could be achieved easily with

format hms_minus3min %tcHH:MM:SS,sss

but not so! we get

invalid %format

I guess we could do it in string format with

gen wanted = subinstr(string(hms_minus3min, "%tcHH:MM:SS.sss"), ".", ",", 1)

But I find it curious that the date-time format does not permit the comma separator for milliseconds.

[daniel klein]

The period is not a separator; it's part of the (fractional) milliseconds. Here, I insert a comma before the (fractional) milliseconds:

. display %tcHH:MM:SS,.sss hms_minus3min[1]
00:02:03,.000

What is indeed a bit puzzling is that the period is not affected by

. set dp comma

. display %tcHH:MM:SS.sss hms_minus3min[1]
00:02:03.000

[Chen Samulsion]

Hemanshu Kumar daniel klein thank you for attention and elaboration.