Need to search for keywords within files modified at a certain time

view story

http://www.unix.com – I have a huge list of files in an Unix directory (around 10000 files). I need to be able to search for a certain keyword only within files that are modified between certain date and time, say for e.g 2012-08-20 12:30 to 2012-08-20 12:40 Can someone let me know what would be the fastest way to perform this search? I tried using find . -exec grep -l "searchword" {} \; and it is taking a lot of time to perform the search as it searches within all the files in the directory. I am not very much familiar with Unix.. Any help you could provide is greatly appreciated. Thanks! (HowTos)