This week many in the U.S. will change their clocks for Daylight Saving Time. Is this a Mitzvah DeOraitha or Takkanah DeRabanan? I can't seem to find any source for this except a poorly worded and weakly sourced question on a website of dubious reliability (see their very obvious disclaimer even warning anyone not to rely on them!) relating to a(n obviously) mythological line in the Pacific Ocean that marks when "tomorrow" begins.

Any solid sources on this?

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4 Answers 4


This is actually a Karaaite custom which messes up the Zemanim.


It is a takkanah for reasons of safety.

The Gemara (Pesachim 112b) states that a black ox during Nissan is especially dangerous. So there is a real problem: if the ox gets used to your daily habits, won't he know where to ambush you and attack? The problem is solved by moving the clock forward one hour sometime during Adar, in preparation for Nissan, so that you're leaving your house an hour earlier than you usually would. And in Berachos 27a, the Gemara states that according to R. Yehudah, האי בקר יתירא להקדים לו שעה אחת - this cattle will be afraid to be an hour early.

(Thanks to @msh210 for the inspiration for that last line.)

  • 1
    This makes me feel better. I was concerned for those who call it "Daylight Savings Time" that they might be 'Over on Bal Tosif. Since it's DeRabanan, I'm guessing it's an acceptable Ḥumrah?
    – Seth J
    Mar 7, 2012 at 2:22
  • @SethJ: going back an hour - and thus gaining one hour - during the fall is indeed a problem of Bal Tosif; indeed, as Gershon said, it's קרא יתירא - the Karaites add.
    – Alex
    Mar 7, 2012 at 2:57
  • Ok, but misspelling (and mispronouncing) it isn't Bal Tosif, right? Is it just a Minhag Ta'uth?
    – Seth J
    Mar 7, 2012 at 3:00
  • @SethJ: it's okay. ודלוגו עלי אהבה - Hashem loves our mispronunciations related to skipping (time).
    – Alex
    Mar 7, 2012 at 3:04
  • +1, but what did I have to do with it?
    – msh210
    Mar 7, 2012 at 3:17

There is an Asmachta - It mentions in the Megilla the "Choshvei Haitim"- the time calculators.

One could ask - Who needs to calculate time as one just needs to look at a watch. Rather, they used to do all the difficult calculations resulting from DST.


It's actually forbidden.

We remind ourselves of this each month when we say

חוק וזמן נתן שלא ישנו

He gave a law and time that shall not change

Additionally, DST is a demonic invention, as we see from the gematriya:

D (4) + S (100) + T (200) = 304 = שד

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