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How can I get the output I want from a lubridate subtraction

Answer 1

+1 A:

You could try using difftime instead, ie:

difftime(syrrupan$Started,syrrupan$dos1,units="days")

Note that this will give you an object of class difftime, if you want a numeric vector, wrap an as.numeric around it. Note also that you can't choose months as an option for units, but you should really stick with a time unit that has a fixed length.

James 2010-09-22 09:03:41

I read with interest the difference in Lubridate between an interval and a period and a duration. Yes, indeed, a month does not have a fixed length. However, for almost all but the most pedantic chronologist a month is equal to 365.25/12 = 30.44 days. I will try difftime later today and report back. Thanks for the suggestion.

Farrel 2010-09-22 11:11:38

In that case you could just divide the above result by 30.44.

James 2010-09-22 11:28:14

difftime is from the base package. Since the columns were "Date" I was also able to simply enter syrrupan$Started-syrrupan$dos1 which created a vector of class "difftime" in days. By the way, the syntax of difftime does not permit the minus sign, rather it should be just a comma. To get months and to have just one significant value after the decimal I used round(as.numeric(difftime(syrrupan$Started,syrrupan$dos1,units="days"))/30.44,1).

Farrel 2010-09-22 17:04:19

difftime is from the base package. Since the columns were "Date" I was also able to simply enter syrrupan$Started-syrrupan$dos1 which created a vector of class "difftime" in days. By the way, the syntax of difftime does not permit the minus sign, rather it should be just a comma. To get months and to have just one significant value after the decimal I used round(as.numeric(difftime(syrrupan$Started,syrrupan$dos1,units="days"))/30.44,1). In lubridate I was able to do this round(as.numeric(as.duration(syrrupan$Started-syrrupan$dos1))/60/60/24/30.44,1) which seems clunky.

Farrel 2010-09-22 17:15:56

Sorry, yes the minus sign crept in when I copied your variable names from the question. I've fixed the answer now.

James 2010-09-23 09:25:05

Answer 2

+1 A:

Hi Farrel,

That's definitely a bug in lubridate. I've made an error report and will fix it for version 0.1:

http://github.com/hadley/lubridate/issues#issue/75

Thanks for bringing it to my attention.

Garrett 2010-09-22 19:15:26

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How can I get the output I want from a lubridate subtraction

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