convert to longer time units, and clock() converts to datetime.įor all of these functions the first argument is a a string or variable containing strings with the date to be converted. The date() function converts strings to dates with days as the time unit weekly(), monthly(), etc. Again, there are a variety of functions available for converting such strings to dates, depending on the time unit desired. It’s also common to have dates stored as strings (text). The result says it’s on January 1, 1960, but you can just ignore that. The nd (numeric date) variables in the example date set are in this form: Converting Numbers to Datesīecause date components stored as separate numbers are easy to use, they’re very common. as that form is easy to convert and unambiguous. If you are planning to make data widely available or to store it for a long time, consider storing dates as separate numbers for year, month, day, etc. Be careful about converting dates from other programs. Note that Excel stores dates as the number of days since Januand some other systems use January 1, 1970. The second step in that process is the tricky one but Stata has a variety of functions that will do it for you, depending on the time unit you want and how the date is currently stored. To create a Stata date variable, you’ll choose a time unit, convert the date to “number of that time unit that have passed since January 1, 1960”, and then apply the appropriate format. It’s really the format that determines what the time unit is. But if it’s formatted with %tm it will be displayed as 3881m5, meaning month 5 (May) of the year 3881. If a variable contains 23,056 and has been formatted with %td, it will be displayed as 15feb2023. The format for datetime variables is called %tc (think ‘clock’). These formats all start with %t followed by a letter corresponding to the time unit: %td for days, %tw for weeks, %tm for months, %tq for quarters, etc. Fortunately, there are special formats for dates that tell Stata to display those numbers as the corresponding human-readable date. But dates as numbers of time units don’t mean much to most humans. For example, if you know a subject’s birthday and the date of the beginning of the study, you can calculate how old they were when the study began by just subtracting. Storing dates as numbers of time units since Janumeans you can do math with them. Always use double precision numbers for datetime variables. Note that this number is too big to store accurately using the default float variable type. When you need to store a date and time (a datetime), the time unit becomes milliseconds. But the time unit could also be weeks, months, quarters, etc., depending on what’s convenient for your project. Stata stores a date as a number: most commonly the number of days that have passed since January 1, 1960. Often your first task is to convert the dates and times in your data set to Stata format, and most of the time everything after that is easy. You’ll notice a few bits of Stata code that are only needed to make that process work well, such as using the ab(30) option with list to prevent variable names from being abbreviated. Note: this article was written as a Jupyter Notebook using the nbstat kernel and then converted to HTML by Quarto. Clear all use https: ///sscc/pubs/stata_dates/dates.dta
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