String date time conversion to numerical date and time variable

[Tim Evans]

Hi all,

I'm using Stata v13.1 in Windows and on a network installation. I have a variable (x526_) containing a string date time value such as

05/01/2014 10:15

I want to split this variable and then generate two extra numeric variables containing 'date' and 'time'.

I can do this using the following code (I loop through a number of variables):

split x526_

foreach var of varlist x526_ {
g `var'_date = date(`var', "DMYhm")
replace `var'_date = date(`var', "DMY") if `var'_date==. & `var'!=""
format `var'_date %td
g `var'_time = clock(`var', "DMYhm")
format `var'_time %tc_HH:MM
}


However, I observe that when generating the time(clock) aspect of the date time variable, Stata is not converting the string date time in an exact form - i.e. it is either adding or subtracting a minute from the old time variable when creating the new time variable - this does not appear to be consistenly applied either. This could have important consequeneces to my analysis. Anyone have a clue what is going on here?


Here are some examples from the editor window:

If I look at the final column shown below (z526__time), the full contents of the entry for the penultimate record is 16jan2012 07:44:12 - and I'm surprised as I don't actually supply this infoirmation from the original string value.

Thanks

Tim

[Nick Cox]

You've broken a cardinal rule: date-times must always be generated as double. What looks like a capriciously appearing bias is just imprecision.

By the way, your loop is a loop over precisely one variable. (Perhaps it's a simplification of original code complicated in a way that you simplified out.)

[Tim Evans]

Nick, Thanks for this. I will go back and generate my date-times as doubles and confirm that resolves my problem. For simplicity, I removed the other variables that are part of the loop.

[Nick Cox]

One of my top tips with date-times, ridiculously simple except that people won't try it!, is to use display for examples where you know the answer.

. di %tc clock("16 Jan 2012 07:45", "DMY hm")
16jan2012 07:45:00

clock() will echo your date-time as you have it, so the error lies elsewhere, and it's in the variable type chosen by default.