Date produced using cofd(td) sometimes, but not always, shows PREVIOUS date + 23 hrs and 59 seconds, instead of date at 00:00. Why?

[Abram Huyser-Honig]

Hi all,

I have a string variable, date_str. I want to get these dates into %tc format to enable merging and/or appending this dataset to another dataset for which the equivalent variable is in %tc format.

I do this in two steps: 1) create date_td, a %td variable, based on date_str
2) create date_tc, a %tc variable, based on date_td

When I generate date_td, using the date() function in Stata 14.2, I get the expected results.

But when I then go on to generate a datetime variable, date_tc, using the cofd() function with date_td, some of the dates in date_tc are moved one second earlier than expected, which shifts the date portion of the tc variable to one whole day earlier than the original variable.


Code

*date_str is a str10 variable containing "YYYY-MM-DD"

gen date_td= date(date_str,"YMD")

format date_td %td

gen date_tc = cofd(date_dt)

format date_tc %tc

Example of results

date_str date_td date_tc dates_match 6/16/2021 16-Jun-21 6/15/2021 23:59 NO 8/12/2021 12-Aug-21 8/12/2021 0:00 YES 8/26/2021 26-Aug-21 8/25/2021 23:59 NO 6/16/2021 16-Jun-21 6/15/2021 23:59 NO 10/6/2021 6-Oct-21 10/6/2021 0:00 YES 5/22/2021 22-May-21 5/22/2021 0:00 YES

Questions
What is going on? Is this expected Stata behavior? If so, is there a way to generate a %tc variable where the date portion will always correspond to the date portion of the %td variable it is based on?

[Nick Cox]

Date-time variables should be generated as double.

See

help datetime

where this is emphasized repeatedly.

[Abram Huyser-Honig]

Thanks Nick!