Put it in the "Variables" tab under the "Globals" tab in Columns UI's Playlist view preferences, then create a column with
$get_global(rel_timestamp)
for the "Display."
You'll need the cwb_hooks component.
And here's some fixed-up code (the one you quoted had major problems). I changed the name of it to "Relative Timestamps" and the global variable to rel_timestamp, because it seems to make a lot more sense to me.
/////////////////////////////////////////
// RELATIVE TIMESTAMPS
// formerly known as natural language timestamps
// by topdownjimmy - v0.5.c - May 6, 2007
/////////////////////////////////////////
// calculate days since last play
$puts(dayssince,$cwb_datediff(%cwb_systemdate%,%last_played%))
// correct "dayssince" to consider early AM to be an extension of previous day
$puts(dayssince,$sub($get(dayssince),$if($and($greater(7,$substr(%cwb_systemdatetime%,12,13)),$greater($substr(%last_played%,12,13),6)),1,0)))
$if($and($greater($substr(%cwb_systemdatetime%,12,13),6),$greater(7,$substr(%last_played%,12,13))),$puts(dayssince,$add($get(dayssince),1)),)
// calculate day of week of last play
$puts(calcdayofweek,$add(1,$mod($add($substr(%last_played%,3,4),$div($substr(%last_played%,3,4),4),$select($substr(%last_played%,6,7),6,2,2,5,0,3,5,1,4,6,2,4),$substr(%last_played%,9,10)),7)))
// correct day of week to consider early AM to be an extension of previous day
$puts(calcdayofweek,$sub($get(calcdayofweek),$ifgreater(7,$substr(%last_played%,12,13),1,0)))
$if($strcmp($get(calcdayofweek),0),$puts(calcdayofweek,7),)
// calculate period of day of last play (night, morning, etc.)
$puts(dayperiod,$select($add(1,$substr(%last_played%,12,13)),Night,Night,Night,Night,Night,Night,Night,Morning,Morning,Morning,Morning,Morning,Afternoon
,Afternoon,Afternoon,Afternoon,Afternoon,Evening,Evening,Evening,Evening,Night,Night,Night))
// calculate natural language timestamp
$if(%last_played%,$puts(rel_timestamp,$ifgreater($get(dayssince),0,$ifgreater($get(dayssince),1,$ifgreater($get(dayssince),6,$if($and($greater($get(dayssince),21),$not($strcmp($left(%cwb_systemdate%,7),$left(%last_played%,7)))),$if($not($strcmp($left(%cwb_systemdate%,4),$substr(%last_played%,1,4))),$select($substr(%last_played%,6,7),January,February,March,April,May,June,July,August,September,October,November,December)' '$ifgreater($get(dayssince),334,$substr(%last_played%,1,4),),$ifgreater(2,$sub($substr(%cwb_systemdate%,6,7),$substr(%last_played%,6,7)),'Last Month',$select($substr(%last_played%,6,7),January,February,March,April,May,June,July,August,September,October,November,December))),$ifgreater($div($add($get(dayssince),4),7),1,$div($add($get(dayssince),4),7)' Weeks Ago',Last Week)),$select($get(calcdayofweek),Sunday,Monday,Tuesday,Wednesday,Thursday,Friday,Saturday)' '$get(dayperiod)),$if($strcmp($get(dayperiod),Night),Last,Yesterday)' '$get(dayperiod)),$if($strcmp($get(dayperiod),Night),Tonight,This $get(dayperiod)))),$puts(rel_timestamp,Never))
$set_global(rel_timestamp,$get(rel_timestamp))
/////////////////////////////////////////