Ruby Time object converted from float doesn't equal to orignial Time object

Question

time = Time.now
fvalue = time.to_f
return time == Time.at(fvalue)

Can somebody here explain why the above expression returns false. How can I create a new Time object from float that matches the original time variable?

Thanks

Answer 1

IEEE 754 double (which is returned by to_f) is not accurate enough to represent the exact time.

t1 = Time.now
f1 = t1.to_f
t2 = Time.at(f1)

# they look the same
t1.inspect #=> '2013-09-09 23:46:08 +0200'
t2.inspect #=> '2013-09-09 23:46:08 +0200'

# but double does not have enough precision to be accurate to the nanosecond
t1.nsec #=> 827938306
t2.nsec #=> 827938318
#                  ^^

# so they are different
t1 == t2 #=> false

Do the following, to preserve the exact time:

t1 = Time.now
r1 = t1.to_r # value of time as a rational number
t2 = Time.at(r1)
t1 == t2 #=> true

Citation from Time.to_r:

This methods is intended to be used to get an accurate value representing the nanoseconds since the Epoch. You can use this method to convert time to another Epoch.