How do you know that the timer used by the select/poll/etc. call has the same resolution?

The variable 'now' should probably be given a more suitable name.

Can the clock resolution be zero? If not, I recommend adjusting the comparisons so that an event scheduled at exactly the rounded-up 'now' value is not considered ready -- it should be strictly before. (But if the clock resolution happens to be zero, an event scheduled at exactly 'now' should be considered ready.)