CODEWITHSUNDEEP
rubytimeutc
keruilin
keruilin

Reputation: 17512

Ruby time operations involving utc

Why does this expression return false?

(Time.now - 10.hours).utc == Time.now.utc - 10.hours

Why accounts for the difference in the results? And which way is correct?

Upvotes: 2

Views: 249

Answers (3)

numbers1311407
numbers1311407

Reputation: 34072

The expressions are equivalent, but when called in sequence, will not return the same result. Note:

Time.now == Time.now #=> false
Time.now - Time.now #=> Some really small negative number

If you call Time.now twice in a row, the 2nd one happens after the first, right? Even if it's a very short amount of time after.

I wouldn't say either form is more right. If you store Time.now and run the same comparison, you get the expected result. 

t = Time.now
t.utc = 10.hours == (t - 10.hours).utc #=> true

Upvotes: 1

Larry K
Larry K

Reputation: 49114

(Time.now - 10.hours).utc.to_s == (Time.now.utc - 10.hours).to_s ==> true

(Time.now - 10.hours).utc - (Time.now.utc - 10.hours) ==> a small, non-zero
                                                          number. IE -1.3e-05

Note that a_time.to_s is the same as a_time.inspect and it only shows the time to the nearest second. But the internal resolution is much smaller than a sec.

Upvotes: 0

Lee Jarvis
Lee Jarvis

Reputation: 16241

Because they're not the same. Rubys Time.now tracks time to a fraction of a second, the output of #to_s just doesn't show that. 

>> Time.now == Time.now
=> false
>> Time.now.to_i == Time.now.to_i  
=> true
>> Time.now.to_f == Time.now.to_f
=> false

Check out the documentation for the Time class here.

Upvotes: 0

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