Stata & date formats

The past few weeks I have been learning about and working with Stata. This program can handle a lot of data and uses commands to edit data or analyse it. There are many commands available and one command is very handy when it comes to changing date formats.
When you work with data and intend to do an event study using the Request Table option from Datastream you may have to change the date format for the event dates to something that Datastream understands (also depends on the date format used by the computer).

In Excel you can use the command =TEXT(A1,”dd-mm-yy”). The Dutch version looks similar: =TEKST(A1;”dd-mm-jj”)
Afterwards you still have to copy and the paste as values to get workable dates for Datastream.

Now if you are working with a dataset in Stata you can use a command to do a similar thing: format [variablename] %tdDD-NN-YY

The elements mean:
format = change the format of a variable
variablename = indicates the date/variable item you wish to change
%tdDD-NN-YY = indicates that the timedate variable should look like 17-02-99. DD = day with two digits, NN = month with two digits and YY = Year with two digits.

Example of dataset before the change:

Now I use the command to change the look of variable dateff: format dateff %tdDD-NN-YY. The result looks as follows:

If you look closely you see that “under water” the variable still looks as it is originally presented: the first date 20-08-07 still looks like 20aug2007. On the right side you also see that dates in Stata are actually variables of the type Integer = a number that can be written without a fractional component (Wikipedia).
Interesting sidenote: in Stata dates are actually integer numbers, just like in Excel. Where in Excel number 1 represents 1-1-1900 in Stata the number 1 represents 1-1-1960. Negative integer numbers in Stata represent older dates. When you work with older dates it is wiser to use 4-digit year formats to avoid confusion.

There are a lot more options to change the dateformat. Another example that may be useful for using with the Event Tool in the CRSP database is:

format variablename %tdCCYYNNDD

In this instance dates will be formatted with 4 digits for a year, for example: 19871022

If you want to know all the options for formatting dates you can use the Help command: help datetime display formats.

Important: If you export a formatted file from Stata to an Excel file you will lose the formatting for the dates you just did. If you wish to keep the dates with the (changed) formatting you need to export/save the file as a (delimited) text file using the command: export delimited filename.txt and then open the text file in Excel using the text wizard. Also remember to open the formatted date column as text to keep the formatted date in tact!

This example applies to Stata editions 13 and 14.


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