That's more difficult, since it's not clear how long a year or a month really is. Is one month equal to 30, 31, 28, 29 or 365.24219/12 days?

Keeping that in mind, here's something I wrote instead of doing useful stuff. It should give rather accurate results in that it works with all the separate numbers (year,month,day,hour,minute,second) instead of converting it all to seconds since some specific date and then using some arbitrary (read: always wrong) period of time for a month, a year etc.
Code:
alias datediff {
  ; calculate difference between date 1 and date 2, in years,months,days,hours,minutes,seconds
  ; date 1 must be before date 2
  ; usage $datediff(dd/mm/yyyy HH:nn:ss, dd/mm/yyyy HH:nn:ss)
  ; meaning $datediff($date $time, $date $time)
  ; example:  [color:green]//echo -a $datediff(3/5/2005 20:00:10, 5/10/2000 12:15:20)[/color]
  ; ** IMPORTANT: $date uses dd/mm/yyyy, if you want to input mm/dd/yyyy style, 
  ; **   add a third parameter (not 0, $null or $false)
  ; example:  [color:green]//echo -a $datediff(1/5/2005 11:15:22pm, 1/8/2005 12:15:22P.M., us)[/color]
  ; hh:nn:ss am/pm is supported (only now I realize how strange it actually is)
  ; my reference: http://www.worldtimezone.com/wtz-names/wtz-am-pm.html
  if ($regex($2,/^0*(\d+)\D0*(\d+)\D(\d+)\D0*(\d+)\D0*(\d+)\D0*(\d+)(?:\D?([ap]\.?m\.?|noon|midday|midnight))?$/i)) {
    var %d1 = $regml($iif($3,2,1)), %m1 = $regml($iif($3,1,2))
    var %h1 = $regml(4), %n1 = $regml(5), %s1 = $regml(6), %y1 = $regml(3)
    if ((%h1 == 12) && (($regml(7) == am) || ($v1 == midnight))) var %h1 = 0
    elseif ((%h1 isnum 1-11) && ($regml(7) == pm)) inc %h1 12
  }
  else return Date 1 in incorrect format
  if ($regex($1,/^0*(\d+)\D0*(\d+)\D(\d+)\D0*(\d+)\D0*(\d+)\D0*(\d+)(?:\D?([ap]\.?m\.?|noon|midday|midnight))?$/i)) {
    var %d2 = $regml($iif($3,2,1)), %m2 = $regml($iif($3,1,2))
    var %h2 = $regml(4), %n2 = $regml(5), %s2 = $regml(6), %y2 = $regml(3)
    if ((%h2 == 12) && (($regml(7) == am) || ($v1 == midnight))) var %h2 = 0
    elseif ((%h2 isnum 1-11) && ($regml(7) == pm)) inc %h2 12
  }
  else return Date 2 in incorrect format
  if ((%m1 !isnum 1-12) || (%d1 !isnum 1-31) || (%h1 !isnum 0-23) || (%m1 !isnum 0-59) $&
    || (%s1 !isnum 0-59)) return Date 1 invalid
  if ((%m2 !isnum 1-12) || (%d2 !isnum 1-31) || (%h2 !isnum 0-23) || (%m2 !isnum 0-59) $&
    || (%s2 !isnum 0-59)) return Date 2 invalid
  var %s = %s1 - %s2
  if (%s < 0) var %s = %s + 60, %n1 = %n1 - 1
  var %n = %n1 - %n2
  if (%n < 0) var %n = %n + 60, %h1 = %h1 - 1
  var %h = %h1 - %h2
  if (%h < 0) var %h = %h + 24, %d1 = %d1 - 1
  var %d = %d1 - %d2
  if (%d < 0) var %d = %d + $daysinmonth(%m1, %y), %m1 = %m1 - 1
  var %m = %m1 - %m2
  if (%m < 0) var %m = %m + 12, %y = %y - 1
  var %y = %y1 - %y2
  if (%y < 0) return reverse the dates please...
  return Difference: %y years %m months %d days %h hours %n minutes %s seconds
}
alias -l daysinmonth {
  ; usage: $daysinmonth(number of month, year)
  ; returns the number of days that month has, with full support for leap years
  if ($1 isin 1.3.5.7.8.10.12) return 31
  elseif ($1 isin 4.6.9.11) return 30
  elseif ($calc($2 % 400) == 0) return 29
  elseif ($calc($2 % 100) == 0) return 28
  elseif ($calc($2 % 4) == 0) return 29
  return 28
}