If you want to find the files modified in the last 24 hours, try:
if you want to find the files modified since yesterday midnight:
find / -daystart -mtime -2
To use the -newer predicate, just touch a temporary file to the date you want to use as threshold, then you will find every file modified at a later time. Example for 28-Oct-2010 13:00:
touch -t 201010281300 /tmp/dummy
find / -newer /tmp/dummy
Hope this helps.