Answered both for me in chrome omnibox on Chrome Canary. Just wait until it flows into your release maybe?

Unable to reproduce the issue on chrome reported version 67.0.3366.0 using Windows-10 with steps mentioned below:
1) Launched chrome reported version and in omnibar entered "venus length of day" & "venus length of rotation"
2) For both got result as 116d 18h 0m.

@Reporter: Please find the attached screenshot for your reference, try to test this issue by creating new person with no apps and extensions in it and let us know if the issue still persists.

Thanks!

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Google Search answers both. The problem is that the Chrome omnibox only answers the former.

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93m4qau783@@, I get both of these in Chrome omnibox too (Chrome 65, in the U.S.).  Sometimes answers are location-sensitive or language-sensitive, even at times when you wouldn't expect it.  What country is your computer located in and what is your interface language?  thanks, mark

Unable to reproduce the issue on Win-7 using chrome reported version #67.0.3366.0 and latest canary #67.0.3370.0.

Attached a screen cast for reference.

Following are the steps followed to reproduce the issue.
------------
1. Typed "venus length of day" into the Chrome omnibox.
2. Chrome provides the correct answer, 116d 18h 0m.
3. Typed "venus length of rotation" into the Chrome omnibox and observed the answer as 16d 18h 0m as expected.

93m4qau783@ - Could you please check the issue on latest canary #67.0.3370.0 by creating a new profile without any apps and extensions and please let us know if the issue still persist or not.
Also please respond to comment #8.

Thanks...!!

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United States, English (US)

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For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

I'm inclined to mark this WontFix for lack of repro., but I'll leave it for the next reviewer to decide.

93m4qau783@, does this still happen today -- I'm wondering if there was a temporary issue (and that may be why others are having trouble reproducing it).

It doesn't answer "venus length of rotation" when first pasted into the omnibox, but does after I delete and retype the last "n". However, that last "n" is bold when it doesn't look like it should be, as you can see in the attached screenshot.

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The inconsistency is unfortunately an aspect of the answering service that we can't immediately change, but it is being improved over time. The last bold character is a known issue, so I'm going to mark this as a duplicate.