[ruby-core:91842] [Ruby trunk Feature#15668] stdlib: Date - Time should return the difference in days

Issue #15668 has been reported by foonlyboy (Eike Dierks).

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Feature #15668: stdlib: Date - Time should return the difference in days
https://bugs.ruby-lang.org/issues/15668

* Author: foonlyboy (Eike Dierks)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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Hi at the ruby/stdlib,

Computing with times and dates is very complicated.
There are so many pitfalls lurking there.

I stumbled about a seemingly simple computation,
involving Date and Time

try:
Date.today - Time.now
=> TypeError: expected numeric

while my naively expected result can done by:
Date.today - Time.now.to_date
=> (0/1)

There must be a whole lot of good reasons why Date - Time is not allowed.

I'd like to suggest:
- improve the error message
- improve the documentation of #-


For the error message:
  TypeError: expected numeric
should be improved to:
  Date - Time is not allowed

The documentation of Date should state why this is so problematic.

You might even want to add a principal documentation,
about the handling of Date and Time in ruby

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