I'm using blockchain.info's address history api function and I'm finding some bogus transaction/block times in the output. Specifically, one transaction has a time stamp of 2005.12.24. Right Christmas Eve, 2005. Now... you KNOW that's not correct! 
The lesson that you should learn from this is that a website named blockchain.info, despite the fancy name, is not a reliable source of information about bitcoin.
After reading this thread and a couple others, I surmise that the miner that generated the block didn't have the time set correctly on the machine. This tells me that these timestamps aren't reliable. So, how do I determine, within a reasonable accuracy of a few minutes, when a transaction actually made it into a block? Is there some independent source of accurate data on this?
The closest that you can get meaningfully, is the timestamp of the block. The timestamp in the block is usually very close* to the actual time that block was found, and is absolutely sure to be within the fuzzy window that the network defines for validity.
You could also run your own service and keep track of the exact instant that you first see each block. However, this will quickly run into the problem of no one giving a fuck when you first saw it. In theory, this problem can be overcome by becoming important in the community, but most people would probably just prefer to use the global timestamp that is already in the block, and accept the limitations thereof.
* And by "very close", I mean within the "reasonable accuracy of a few minutes" that you are asking for.