Time Zone Handling
I just want to clarify, even though this has been commented so future people don't miss this very important distinction.
DateTime.strptime("1318996912",'%s') # => Wed, 19 Oct 2011 04:01:52 +0000
displays a return value in UTC and requires the seconds to be a String and outputs a UTC Time object, whereas
Time.at(1318996912) # => 2011-10-19 00:01:52 -0400
displays a return value in the LOCAL time zone, normally requires a FixNum argument, but the Time object itself is still in UTC even though the display is not.
So even though I passed the same integer to both methods, I seemingly two different results because of how the class' #to_s method works. However, as @Eero had to remind me twice of:
Time.at(1318996912) == DateTime.strptime("1318996912",'%s') # => true
An equality comparison between the two return values still returns true. Again, this is because the values are basically the same (although different classes, the #== method takes care of this for you), but the #to_s method prints drastically different strings. Although, if we look at the strings, we can see they are indeed the same time, just printed in different time zones.
Method Argument Clarification
The docs also say "If a numeric argument is given, the result is in local time." which makes sense, but was a little confusing to me because they don't give any examples of non-integer arguments in the docs. So, for some non-integer argument examples:
Time.at("1318996912")
TypeError: can't convert String into an exact number
you can't use a String argument, but you can use a Time argument into Time.at and it will return the result in the time zone of the argument:
Time.at(Time.new(2007,11,1,15,25,0, "+09:00"))
=> 2007-11-01 15:25:00 +0900
Benchmarks
After a discussion with @AdamEberlin on his answer, I decided to publish slightly changed benchmarks to make everything as equal as possible. Also, I never want to have to build these again so this is as good a place as any to save them.
Time.at(int).to_datetime ~ 2.8x faster
09:10:58-watsw018:~$ ruby -v
ruby 2.3.7p456 (2018-03-28 revision 63024) [universal.x86_64-darwin18]
09:11:00-watsw018:~$ irb
irb(main):001:0> require 'benchmark'
=> true
irb(main):002:0> require 'date'
=> true
irb(main):003:0>
irb(main):004:0* format = '%s'
=> "%s"
irb(main):005:0> times = ['1318996912', '1318496913']
=> ["1318996912", "1318496913"]
irb(main):006:0> int_times = times.map(&:to_i)
=> [1318996912, 1318496913]
irb(main):007:0>
irb(main):008:0* datetime_from_strptime = DateTime.strptime(times.first, format)
=> #<DateTime: 2011-10-19T04:01:52+00:00 ((2455854j,14512s,0n),+0s,2299161j)>
irb(main):009:0> datetime_from_time = Time.at(int_times.first).to_datetime
=> #<DateTime: 2011-10-19T00:01:52-04:00 ((2455854j,14512s,0n),-14400s,2299161j)>
irb(main):010:0>
irb(main):011:0* datetime_from_strptime === datetime_from_time
=> true
irb(main):012:0>
irb(main):013:0* Benchmark.measure do
irb(main):014:1* 100_000.times {
irb(main):015:2* times.each do |i|
irb(main):016:3* DateTime.strptime(i, format)
irb(main):017:3> end
irb(main):018:2> }
irb(main):019:1> end
=> #<Benchmark::Tms:0x00007fbdc18f0d28 @label="", @real=0.8680500000045868, @cstime=0.0, @cutime=0.0, @stime=0.009999999999999998, @utime=0.86, @total=0.87>
irb(main):020:0>
irb(main):021:0* Benchmark.measure do
irb(main):022:1* 100_000.times {
irb(main):023:2* int_times.each do |i|
irb(main):024:3* Time.at(i).to_datetime
irb(main):025:3> end
irb(main):026:2> }
irb(main):027:1> end
=> #<Benchmark::Tms:0x00007fbdc3108be0 @label="", @real=0.33059399999910966, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.32000000000000006, @total=0.32000000000000006>
****edited to not be completely and totally incorrect in every way****
****added benchmarks****