I couldn't quite think of an intelligible title for this article so I've decided to run with the PHP function name. The function takes a Unix timestamp as a parameter and returns a nicely formatted textual description of how long ago that was, e.g. 2 weeks ago, hence the unimaginative title, x y ago where 'x' is the number, 'y' is the unit and 'ago' is, eh, ago. You see this date difference functionality all over the web. It's particularly popular in forums where you often see headers of the form 'Posted 4 minutes ago by duke'.

The function is strongly related to VBScript's DateDiff, the difference being that DateDiff returns the number of intervals between 2 dates whereas xyago automatically assumes the second date's value as now. You can pass in a different value for $dateto but the result wouldn't really make a whole lot of sense given that 'ago' has quite a specific meaning in the English language. I've included the ability to pass in the second date simply if you want to modify this function to operate more like DateDiff. The second difference between this function and DateDiff is that with DateDiff the interval is specified whereas with xyago the interval is determined.

The general logic is as follows: If $datefrom is not specified or is equal to '0', just return "A long time ago" as it's probably an error rather than someone having posted a message at the exact second of the epoch. Just playing the statistics here with that line but you can actually return the correct value, i.e. 37 years ago or whatever by uncommenting the line.

Firstly, we determine the difference between the 2 dates in terms of the number of seconds. We then send that value down an if-else ladder looking for the most appropriate interval. This is to determine the 'y' of the function return, i.e. not how many of the units, but what are the actual units. If the difference between the 2 timestamps is less than 60 seconds, seconds seems an approriate interval, similarly, if the difference is between 60 and 60*60 (an hour), minutes is an approriate interval. It continues along like that until it gets to between 7 and 30 days. Less than 7, we set the interval to be 'days', greater than or equal to 30, we set the interval to be 'months', in between is 'weeks'. 30 was chosen reasonably arbitrarily given that there aren't, of course, 30 days in every month, it just makes it possible to return '4 weeks ago'.

In this sense, the function will return an 'incorrect' value if the difference between the 2 timestamps is in this region. It's not exact, but if you wanted to print out the exact time that something happened you would probably just use date("r",$mytimestamp). The nature of this function is that it obfuscates to some degree the exact time that something happened.

Ok, here's the function:


function xyago($datefrom,$dateto=-1)
{
// Defaults and assume if 0 is passed in that
// its an error rather than the epoch

if($datefrom==0) { return "A long time ago"; }
if($dateto==-1) { $dateto = time(); }

// Calculate the difference in seconds betweeen
// the two timestamps

$difference = $dateto - $datefrom;

// If difference is less than 60 seconds,
// seconds is a good interval of choice

if($difference < 60)
{
$interval = "s";
}

// If difference is between 60 seconds and
// 60 minutes, minutes is a good interval

elseif($difference >= 60 && $difference<60*60)
{
$interval = "n";
}

// If difference is between 1 hour and 24 hours
// hours is a good interval

elseif($difference >= 60*60 && $difference<60*60*24)
{
$interval = "h";
}

// If difference is between 1 day and 7 days
// days is a good interval

elseif($difference >= 60*60*24 && $difference<60*60*24*7)
{
$interval = "d";
}

// If difference is between 1 week and 30 days
// weeks is a good interval

elseif($difference >= 60*60*24*7 && $difference < 60*60*24*30)
{
$interval = "ww";
}

// If difference is between 30 days and 365 days
// months is a good interval, again, the same thing
// applies, if the 29th February happens to exist
// between your 2 dates, the function will return
// the 'incorrect' value for a day

elseif($difference >= 60*60*24*30 && $difference < 60*60*24*365)
{
$interval = "m";
}

// If difference is greater than or equal to 365
// days, return year. This will be incorrect if
// for example, you call the function on the 28th April
// 2008 passing in 29th April 2007. It will return
// 1 year ago when in actual fact (yawn!) not quite
// a year has gone by

elseif($difference >= 60*60*24*365)
{
$interval = "y";
}

// Based on the interval, determine the
// number of units between the two dates
// From this point on, you would be hard
// pushed telling the difference between
// this function and DateDiff. If the $datediff
// returned is 1, be sure to return the singular
// of the unit, e.g. 'day' rather 'days'

switch($interval)
{
case "m":
$months_difference = floor($difference / 60 / 60 / 24 / 29);
while (mktime(date("H", $datefrom), date("i", $datefrom), date("s", $datefrom), date("n", $datefrom)+($months_difference), date("j", $dateto), date("Y", $datefrom)) < $dateto)
{
$months_difference++;
}
$datediff = $months_difference;

// We need this in here because it is possible
// to have an 'm' interval and a months
// difference of 12 because we are using 29 days
// in a month

if($datediff==12)
{
$datediff--;
}

$res = ($datediff==1) ? "$datediff month ago" : "$datediff months ago";
break;

case "y":
$datediff = floor($difference / 60 / 60 / 24 / 365);
$res = ($datediff==1) ? "$datediff year ago" : "$datediff years ago";
break;

case "d":
$datediff = floor($difference / 60 / 60 / 24);
$res = ($datediff==1) ? "$datediff day ago" : "$datediff days ago";
break;

case "ww":
$datediff = floor($difference / 60 / 60 / 24 / 7);
$res = ($datediff==1) ? "$datediff week ago" : "$datediff weeks ago";
break;

case "h":
$datediff = floor($difference / 60 / 60);
$res = ($datediff==1) ? "$datediff hour ago" : "$datediff hours ago";
break;

case "n":
$datediff = floor($difference / 60);
$res = ($datediff==1) ? "$datediff minute ago" : "$datediff minutes ago";
break;

case "s":
$datediff = $difference;
$res = ($datediff==1) ? "$datediff second ago" : "$datediff seconds ago";
break;
}
return $res;
}


That's it, the function can of course be modified to return difference phrasing or difference results based on different intervals. The scope of this article is simply to provide a framework from where you can determine how many of some particular unit of time have passed. To utilise this function, you could call the following:


$now = time();

// Prints '5 seconds ago'
print xyago(mktime(
date("H",$now),date("i",$now),
date("s",$now)-5,date("m",$now),
date("d",$now),date("Y",$now)))."\r\n";

// Prints '5 minutes ago'
print xyago(mktime(
date("H",$now),date("i",$now)-5,
date("s",$now),date("m",$now),
date("d",$now),date("Y",$now)))."\r\n";

// Prints '5 hours ago'
print xyago(mktime(
date("H",$now)-5,date("i",$now),
date("s",$now),date("m",$now),
date("d",$now),date("Y",$now)))."\r\n";

// Prints '5 days ago'
print xyago(mktime(
date("H",$now),date("i",$now),
date("s",$now),date("m",$now),
date("d",$now)-5,date("Y",$now)))."\r\n";

// Prints '2 weeks ago'
print xyago(mktime(
date("H",$now),date("i",$now),
date("s",$now),date("m",$now),
date("d",$now)-14,date("Y",$now)))."\r\n";

// Prints '4 weeks ago'
print xyago(mktime(
date("H",$now),date("i",$now),
date("s",$now),date("m",$now),
date("d",$now)-30,date("Y",$now)))."\r\n";

// Prints '1 month ago'
print xyago(mktime(
date("H",$now),date("i",$now),
date("s",$now),date("m",$now),
date("d",$now)-31,date("Y",$now)))."\r\n";

// Prints '11 months ago'
print xyago(mktime(
date("H",$now),date("i",$now),
date("s",$now),date("m",$now),
date("d",$now)-364,date("Y",$now)))."\r\n";

// Prints '1 year ago'
print xyago(mktime(
date("H",$now),date("i",$now),
date("s",$now),date("m",$now),
date("d",$now)-365,date("Y",$now)))."\r\n";


Due to the messy nature of months, this function can and will return incorrect values if leap years are involved, if you require a more accurate representation of a timestamp, use a function call like the following: date("r",$mytimestamp). If you need xyago to return really accurate values, the following points might prove useful:

1) Ensure that all timestamps are in UTC (Universal Time Coordinated) or failing that, at least ensure all timestamps are of the same timezone.

2) Don't rely on there being 86400 seconds in a calendar day, particularly in any non-UTC timezone. It's technically incorrect to assume this and if you need to add a day to a Unix timestamp, use $i = strtotime("+1 day", $i) instead of $i += 86400.

3) It goes without saying that if you can't be sure there are 86400 seconds in a calendar day, don't assume there are 31536000 seconds in a calendar year &c. Calendar time is surprisingly complex with irregular length months, leap years, leap seconds, ephemeris time &c. so just because Google tells you there are 31,556,926seconds in a year, don't necessarily believe them!

4) Even absorption spectroscopy atomic clocks can't measure time exactly without synchronisation, so don't be surprised if your PHP code misses a beat here and there!

That's it, hope this function proves useful.

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