Feature #16745
open
Improving Date and DateTime comparison
Description
Currently, when a Date and DateTime are compared, the Date is treated as a DateTime with time "00:00:00 +0000". This results in behavior which could be surprising when the other DateTime is not in UTC ("+0000"):
date = Date.new(2020, 1, 1) # 2020-01-01
datetime1 = DateTime.new(2020, 1, 1, 0, 0, 0, "+0100") # 2020-01-01 00:00:00 +0100
datetime2 = DateTime.new(2020, 1, 1, 1, 0, 1, "+0100") # 2020-01-01 01:00:01 +0100
date <=> datetime1 # 2020-01-01 00:00:00 +0000 <=> 2020-01-01 00:00:00 +0100
# == 1
date <=> datetime2 # 2020-01-01 00:00:00 +0000 <=> 2020-01-01 01:00:01 +0100
# == -1
I think it would be less surprising if the comparison used the offset of the other DateTime:
date <=> datetime1 # 2020-01-01 00:00:00 +0100 <=> 2020-01-01 00:00:00 +0100
# == 0
date <=> datetime2 # 2020-01-01 00:00:00 +0100 <=> 2020-01-01 01:00:01 +0100
# == -1
Or, another possibility is to go one step further and convert the other DateTime to Date:
date <=> datetime1 # 2020-01-01 <=> 2020-01-01
# == 0
date <=> datetime2 # 2020-01-01 <=> 2020-01-01
# == 0
This last behavior leans on the idea that a Date is semantically a range of DateTimes, instead of a single DateTime with its time omitted. It would also cause (date <=> datetime) == 0 when (date === datetime) == true.
Of course, these changes may break backwards compatibility, and so they may not be feasible. But, related issues have been discussed in rails/rails#36462 and rails/rails#36579, and fixing this behavior in Ruby itself would solve them.
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