The Hebrew date comes from your device's own calendar, and the Jewish day begins at nightfall,
so after nightfall the site already shows tomorrow's date and says so. Nightfall itself, and
every other time on the Today page, is computed on your device from sunrise and sunset for your
city. No server is involved, so the times work with no signal.
The definitions are the ones Chabad.org publishes in "About Our Zmanim Calculations":
| Dawn (alot hashachar) | sun 16.9° below the horizon |
| Earliest tallit and tefillin (misheyakir) | 10.2° |
| Sunrise and sunset | 0.833°, sea level |
| True sunrise and sunset (the Alter Rebbe) | 1.583° |
| A proportional hour | (true sunset − true sunrise) ÷ 12 |
| Latest Shema | true sunrise + 3 proportional hours |
| Latest Shacharit | + 4 |
| Midday | + 6 |
| Earliest Mincha | + 6.5 |
| Plag hamincha | true sunset − 1.25 |
| Nightfall (tzeit hakochavim) | 6° |
| Shabbat ends, Yom Tov ends, fast ends | 8.5° |
| Candle lighting | 18 minutes before sunset |
The sun calculation is the standard NOAA one and agrees with published tables to about a
minute at the center of a city. At the far edge of a large metro it can be two or three minutes
off, so for Shabbat candle lighting go by the time your own community publishes. There is no
elevation adjustment. Far enough north that the sun never gets 16.9° below the horizon in
summer, the missing times fall back to fixed minutes and the Today page says so.
Your city is guessed from your device's time zone. If the guess is wrong, "change city" under
the times on the Today page offers a list, or your device's own location, rounded so it does
not identify your house. The molad announced on Shabbat Mevarchim and the Kiddush Levana window
are computed from the traditional molad reckoning and match the standard published values.