In an array of dates how to find closest previous date from a given date?

Question

For example I have an array of 1000 dates for example 1-feb-2010,15-jan-1997,10-feb-2001,17-feb-2010 and so on....If date given is for exampl 15-feb-2010 I need to find closest previous date from given date. In this case correct answer should be 1-feb-2010. How to find this date? Thanx for help