Post Time Stamp Issue

I posted a post on my blog about 5 days ago, but now that I try to update the date to todays date, the post does not show up anymore.

Basically I switched the date from the 17th to the 22nd (clicked edit timestamp). Then when i refresh my blog, “Sorry, but you are looking for something that isn’t here.” shows up. I am using the default theme in 1.5

why does it do this? can i not update my timestamp?