The webcomic xkcd is an extremely popular webcomic in online nerd spaces. It updates thrice every week: on Mondays, Wednesdays and Fridays. Due to this regularity, it's possible to predict the date of a webcomic number well in advance. For example, I managed to manually predict #3000 several months beforehand (which, by the way, is why I started this whole project), although you could've predicted as far back as #1663, over eight years before #3000 released. This calculator computes the release date of a comic using its index, based on xkcd's regularity.
The "retrodictions" get more inconsistent as you go back in time. This calculator assumes that xkcd #1 was released on Monday, August 22nd 2005 (it actually released a month later on Friday, September 30th) and that Randall has kept his modern pace at any point in history, including backwards in time, which, yes, you can input negative values. Even then, there are various comics not posted on schedule after #1633, however they have always been balanced by waiting longer or shorter to release a new one.
The source code is at script.js. Note that this calculator is slightly inaccurate for xkcds extremely far into the future as Javascript uses floating point arithmetic for any number, generating inaccuracies beyond around #253 (#1016). Since these are "supposed" to be made hundreds of trillions of years in the future, well beyond human, including Randall's, lifespan,[citation needed] these inaccuracies don't matter if you're simply using this for predicting xkcds in the near future. If you really do need to predict xkcds without these inaccuracies, I have also written a Python version: xkcd.py, which uses arbitrary integer sizes.
Since you're inputting comic indices in the far past, it's worth noting that this calculator uses the proleptic Gregorian calendar.
Short date:
Number:
Date: -- Invalid date! Dates are in YMD format.