Please check this page before emailing me so I can avoid answering questions that I've already answered here.
(Items recently added are marked with
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Hebrew calendar calculations
Sorry, but I prefer to reserve my rights and options. Besides, the quality of my code is embarrasing.
Other platforms for Kaluach
If you REALLY, REALLY want a Hebrew calendar and/or halachic times on your web site, you can take a look at Abu Mami's Javascript programs. Click on one of the links under Scripts on the navigation bar (on the left side of the screen). There are scripts for a monthly calendar, halachic times, or to simply display the Hebrew date on your web site.

There is a web-based version of Kaluach available
at www.kaluach.net.
Registration
(generic answer to all variations) I don't check on who donates or not. However, the condition that I make for "official" registration is a donation to my beit knesset. If you are using Kaluach without registering it, you are halachically in deep doo-doo.
(sympathetic answer to variation 1) I appreciate your problem. Unfortunately I don't have a solution to this. You can send the money whenever you have the appropriate opportunity (eg. you or a friend coming to Israel).
(specific answer to variations 2 and 3) I don't believe in buying products from multi-trillion dollar monopolies, but hey, if I don't pay, I don't get the product I want. Same with Kaluach. My condition for the legal use of Kaluach is a charitable donation to my beit knesset. No payment, no legal use of Kaluach. Instead of pocketing the proceeds for myself, I decided that the money would go to a good cause. Seems fair enough to me.
User Interface
Printing
If you are using version 2, then make sure your printer has ink or toner.
Halachic issues
Many siddurim are based on older works. For example, my copy of the Shilo Siddur (a siddur commonly used in America) states that asking for rain commences on December 4. This would be correct for the 19th century. Well, almost correct, since no mention is made of years that fall before a civil leap year, which would begin Dec 5. However, in the 20th century, the correct date would be the 5th of December, and the 6th of December in years that fall before a leap year. This is because the date of asking for rain is calculated according to Rav Shmuel's tekufot, which correspond to the Julian calendar. This means that the dates slide forward as the years progress. In the 22nd century, the correct dates for asking for rain in the diaspora will the 6th and 7th of December.
[important note]: It is a minhag in Israel to use the tables of halachic times as compiled by Rav Tucazinski z"l. These tables were derived by sightings of the sun (as opposed to the calculated times in Kaluach and other programs) and are accurate only for Jerusalem and Bnei Brak. Using these tables with adjustments for other cities in Israel is wrong, since the times in these tables are sighted times, not calculated times. These tables are only applicable for Jerusalem and Bnei Brak. For that matter, these tables will eventually shift out of accuracy as the years go by, since the sunrise/sunset times are based on the solar calendar, and the times shift slightly as the years/centuries progress.
Odds and ends
(Copyright © 5761 (2001 CE). May not be reproduced in part or whole without written consent of Yisrael Hersch)