[Replicant]

In the node table, you will find columns named 'created' and 'publishdate'. They are unix timestamps. Changing these will change the dates shown in the posts. There are other timestamps that need to be changed. Any dates in other tables such as nodevisit, nodestats, nodevotes, picturecomment, picturecomment_hash, pollvote, postedhistory.........and so forth that correspond to that nodeid should also be changed. This is where a php script script would come in handy. I've changed dates on posts before in just the node table with no direct issues. But for an entire site, there may be underlying issues with search functions, edit functions and administration issues. Any function that relies on timestamps could directly be affected. There are at least 50 tables which have a dateline column in them.

How many posts, threads and channels do you have? There may be other options.