Try the below script! Make sure to update the correct date time format variable in the set $convtdApprovalTimestamp variable piece as per the date time format of the data you've in the field "ApprovalTimestamp". Currently, it's set to $ISO8601DateTime.
Also, instead of looping through entire list and checking whether the difference is less than 12 hrs before printing the Record.Name, you can also print data until the loop encounters the first record where the difference becomes > 12 hours; #break could help breaking out of the loop as soon as the difference becomes > 12 hours. This should work as we're sorting the list in descending order as per ApprovalTimestamp field's value".
You should also check out Sandy's Velocitips: Switch email content based on day/time blog post to understand how to deal with the date/time-responsive content in general.
#set( $defaultTimeZone = $date.getTimeZone().getTimeZone("America/New_York") )
#set( $defaultLocale = $date.getLocale() )
#set( $calNow = $date.getCalendar() )
#set( $ret = $calNow.setTimeZone($defaultTimeZone) )
#set( $calConst = $field.in($calNow) )
#set( $ISO8601DateOnly = "yyyy-MM-dd" )
#set( $ISO8601DateTime = "yyyy-MM-dd'T'HH:mm:ss" )
#set( $ISO8601DateTimeWithSpace = "yyyy-MM-dd HH:mm:ss" )
#set( $ISO8601DateTimeWithMillisUTC = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'" )
#set( $ISO8601DateTimeWithMillisTZ = "yyyy-MM-dd'T'HH:mm:ss.SSSZ" )
#set ($sortedUpdated = $sorter.sort($appRecord_cList,"ApprovalTimestamp:desc"))
#foreach($Record in $sortedUpdated)
#if (!($Record.ApprovalTimestamp.isEmpty()))
#set( $convtdApprovalTimestamp=$convert.toCalendar(
$convert.parseDate(
$Record.ApprovalTimestamp,
$ISO8601DateTime,
$defaultLocale,
$defaultTimeZone
)
))
#if($date.difference($convtdApprovalTimestamp,$calNow).getHours() > 12)
#break
#end
${Record.Name}
#end
#end