Between books, magazines, websites, and your friendly neighborhood pediatrician, there are tons of resources to help guide new parents through the development of their new bundles of joy. You can track your kiddo's progress toward various typical milestones and even predict periods of increased fussiness and sleep regression, as long as you know how many days, weeks, or months old your little darling is. But with a new kid in the house, you're sleep deprived out of your mind and barely know which way is up, and it's not like any of us are really used to measuring age at that level of granularity.
Once upon a time I came up with a solution that worked for me--I wrote a little program that would drop an RSS feed every day with my daughter's age in it. I got a couple of suggestions that it would make a useful web service, and though I agreed, it took me a long time to finally get around to it.
Lucky for you, the wait is over! HowOldIsMyKid.com is live and ready to help you keep track of exactly how long your munchkin has been around. Just plug in your kid's name and birthdate, hit the button, and optionally subscribe to the RSS feed so that you'll get daily updates--and then it's a quick trip to being Parent of the Year.
I hope you enjoy it--please let me know what you think!
(Aside: I'm very happily posting this live from the stage at PyCon--it's been a great conference and I can't recommend it enough!)