How about a brute force space-for-time tradeoff?

A 3.8Mb bitmap can represent an entire year's worth of seconds. Allocate and clear the bitmap, then turn on bits for second within intervals. The probe to determine whether a given time occurs within at least one interval is O(1).

This merges intervals, so it's not useful if you're trying to determine which intervals a given time occurs within. A tree structure is more appropriate for that.

"It's not the Zen way, but it gets the job done." --Garrison Keillor