A Time is a number of seconds since an epoch whereas a DateTime is a number of days since an epoch which is why adding 1 to a DateTime adds a whole day. You can however add fractions of a day, for example
d = DateTime.now
d + Rational(10, 86400)
Will add 10 seconds to d (since there are 86400 seconds in a day).
If you are using Rails, Active Support adds some helper methods and you can do
d + 20.minutes + 10.seconds
Which will do the right thing is d is a DateTime or a Time. You can use Active Support on its own, and these days you can pull in just the bits you need. I seem to recall that this stuff is in activesupport/duration. I believe there are a few other gems that offer help with time handling too.