Here's the version with the bugfix.

Code:
alias fmtdur {
  ; Usage: $fmtdur(secs [, format])
  ; Formatting: [y year] [m month] [w week] [d day] [h hour] [n minute] [s second] [u units]
  ; Using the same character consecutively will zeropad that unit to the given length (ie. n = 7, nnn = 007)
  ; Doubling the u character will cause it to use the full format (ie. nu = 6mins, nuu = 6minutes)
  ; Uppercase U characters will capitalize units.
  var %fmt = $iif($2,$2,wu du hu nu su), %secs = $int($1), %count = $regex(fmtdur, %fmt, /(([^ymwdhns]*)(y+|m+|w+|d+|h+|n+|s+)([^ymwdhnsUu]*)([Uu]*))/g)
  var %yrs, %mnths, %wks, %days, %hrs, %mins, %i = 1, %fmtchars, %unitchars, %durstr, %amount, %units
  if (y isincs %fmt) { %yrs = $int($calc(%secs / 31557600)) | %secs = $calc(%secs % 31557600) }
  if (m isincs %fmt) { %mnths = $int($calc(%secs / 2629800)) | %secs = $calc(%secs % 2629800) }
  if (w isincs %fmt) { %wks = $int($calc(%secs / 604800)) | %secs = $calc(%secs % 604800) }
  if (d isincs %fmt) { %days = $int($calc(%secs / 86400)) | %secs = $calc(%secs % 86400) }
  if (h isincs %fmt) { %hrs = $int($calc(%secs / 3600)) | %secs = $calc(%secs % 3600) }
  if (n isincs %fmt) { %mins = $int($calc(%secs / 60)) | %secs = $calc(%secs % 60) }
  while %i <= $calc(%count * 5) {
    %fmtchars = $regml(fmtdur, $calc(%i + 2))
    %unitchars = $regml(fmtdur, $calc(%i + 4))
    %amount = $charpad($replace($left(%fmtchars, 1),y,%yrs,m,%mnths,w,%wks,d,%days,h,%hrs,n,%mins,s,%secs), $len(%fmtchars), 0)
    if (u isin %unitchars) {
      if ($len(%unitchars) == 1) %units = $replacex($left(%fmtchars,1),y,yr,m,mth,w,wk,d,dy,h,hr,n,min,s,sec)
      else %units = $replacex($left(%fmtchars,1),y,year,m,month,w,week,d,day,h,hour,n,minute,s,second)
      if (%amount != 1) %units = $+(%units, s)
      if (U isincs %unitchars) %units = $+($upper($left(%units,1)), $right(%units,-1))
    }
    else %units = $null
    %durstr = $+($left(%durstr, -1), $regml(fmtdur, $calc(%i + 1)), %amount, $regml(fmtdur, $calc(%i + 3)), %units, x))
    inc %i 5
  }
  return $left(%durstr, -1)
}
alias charpad {
  ; Usage: $charpad(string, padwidth, character)
  return $iif($len($1) < $int($2), $+($str($left($$3,1),$calc($int($2) - $ifmatch)),$1), $1)
}


Edit (5 minutes later): Just fixed one more bug and cleaned up a tiny bit of code.

Spelling mistakes, grammatical errors, and stupid comments are intentional.