strtotime() – is it useful?

Every now and then I get e-mails with questions that ask “How can I add X days to a given date?”, “How can I figure the day that corresponds to next thursday?”, and others along the same line. It scares me when I see replies that include enormous codes that execute innumerous function even including some bizarre leap year determination algorithms, I just can’t understand why all the complication and fuss.

The strtotime() function exists to solve these problems and i plan to introduce you to it and show a few usage examples. Also I’m going to check function performance using a simples benchmark comparison.

What’s this function’s secret? Well its simples, it accepts a string, in “US English date” format, and parses it, generating a unix timestamp. This simple ability expands the horizon of functionality making it possible to add dates, get dates of specific days and many other uses.

It accepts two parameter, time and now:
time = String representation in GNU Date Format
now – timestamp reference in a unix timestamp format

How do I use it?

The function can be used only with the time parameter, but the presence of the now parameter make it use that timestamp as a reference, manipulating that date instead of the current date. Let’s see some examples:

< ?
//Get current date/time
echo strtotime("now");

//Using a string date
echo strtotime("10 September 2000");

//Adding one day
echo strtotime("+1 day");

//Adding a week
echo strtotime("+1 week");

//Addingone week, tow days, four hours and two seconds
echo strtotime("+1 week 2 days 4 hours 2 seconds");

//Seek using weekday names, next thursday
echo strtotime("next Thursday");

//Seek using weekday names, previous monday
echo strtotime("last Monday");

//Get today’s date and add ten days to it
//note the use of the now parameter
$now = strtotime("now");
echo strtotime("+10 day",$now);


Its important to note the syntax “+3 day” where the plus sign must be immediately followed by the number, no spaces, and day must be in the singular, not plural. This can cause a few problems as I already verified in a local PHP discussion list.

I don’t know if this article may help a lot of you folks, it’s all in the manual, but at least I’ll save some people from surrendering to custom functions that are very little flexible in input format, or complex classes that add dozens of extra code lines.

I should also note that in countries that do not use that standard “YYYY-MM-DD”, as in Brazil, this function has another important ability. Since MySQL uses this standard, when reading from databases we cant just throw the date variable to a date() or strftime() function, without passing through a mktime() ritual or some other “string-type” manipulation. Strtotime() lets you get passed that rather easily, like this:

< ?
//$date_from_db has a MySQL format date
echo date('d/m/Y'.strtotime($date_from _db));

In my head by this point only one point needed further analysis, the performance. Is this function faster or slower that the common household custom function? Well I decided to benchmark it and get some answers, like these:


//Custom function
$dateInit = "01/08/2006 08:04:20";
date("d/m/Y H:i:s", dateAdd($dateInit, +15, "day"));

//Using strtotime
$ dateInit = "01/08/2006 08:04:20";
date("d/m/Y H:i:s", strtotime("+15 day",strtotime($dateInit)));

Executing 100 times
Benchmark functiono: 0.000267641544342
Benchmark strtotime(): 0.000428168773651

That result was really not what I was expecting, a 0.0002 delay relative to the custom function really felt like a bucket of cold water, so I decided to check for something out of place. That’s when I realized that using the initial date above (dd-mm-yyyy format – brazilian), we need to execute strtotime tow time instead of just one custom function call. So I decided to try using a timestamp as the input date. This time around it strtotime superior performance became clear as the custom function had to convert the timestamp to a regular date before executing the addition.

Using a Textual Date
Executing 100 times
Benchmark function: 0.000275664329529
Benchmark strtotime(): 0.000425822734833

Using timestamp
Executing 100 times
Benchmark function: 0.000400955677032
Benchmark strtotime(): 0.000323491096497

Benchmark source-code: here
Live benchmark: here

Well, even with the lesser performance using a textual date, cause by the parsing process, the ease of use and variety of use is clearly superior, witch leads to simpler code e less lines of code.

That leaves the choice up to y’all, simplicity and flexibility or stiffness and slight better performance?

This post is also available in: Portuguese (Brazil)

2 thoughts on “strtotime() – is it useful?

  1. Strtotime is a very useful function. Keep in min, If you’re writing an application that deals with date + time values across multiple time zones, make sure all your timestamps are for GMT (basically all the same timezone, but GMT is the standard) so that you don’t have weird output values (off by too many or few hours than what you expected).

  2. Beware also that this does not work well for months, as it tries to change month and leave day the same which does not work well when going from e.g. 31 day month to 30 day month. For example:

    echo(date(‘d/M/Y’,strtotime(‘-1 month’,mktime(12,0,0,7,31,2007))));
    echo(date(‘d/M/Y’,strtotime(‘+1 month’,mktime(12,0,0,5,31,2007))));

    both of which yield 1st July. Explanation here:

    “The fuzz in units can cause problems with relative items. For example, `2003-07-31 -1 month’ might evaluate to 2003-07-01, because 2003-06-31 is an invalid date.”

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