Block post updates and deletion after a set period

From the PHP manual acceptable units for strtotime() are:

unit    (('sec' | 'second' | 'min' | 'minute' | 'hour' | 'day' | 'fortnight' | 'forthnight' | 'month' | 'year') 's'?) | 'weeks' | daytext

Try using -300 second (not seconds).