Try using foo_dynfil. It probably is the cleanest way of calculating date-dependent field values.

/////////////////////////////////////////
// RELATIVE TIMESTAMPS
// formerly known as natural language timestamps
// by topdownjimmy - v0.5.c - May 6, 2007
// modified for foo_dynfil by ojdo - June 11, 2012
/////////////////////////////////////////

// calculate days since last play
$puts(dayssince,$date_diff(%last_played%))

// correct "dayssince" to consider early AM to be an extension of previous day
$puts(dayssince,$sub($get(dayssince),$if($and($greater(7,$substr(%now%,12,13)),$greater($substr(%last_played%,12,13),6)),1,0)))
$if($and($greater($substr(%now%,12,13),6),$greater(7,$substr(%last_played%,12,13))),$puts(dayssince,$add($get(dayssince),1)),)

// calculate day of week of last play
$puts(calcdayofweek,$day_of_week(%last_played%))

// 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($left(%now%,7),$left(%last_played%,7)))),$if($not($strcmp($left(%now%,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($substr(%now%,6,7),$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))

// and display
$get(rel_timestamp)