Even if the limitation period ran out for Clapper on the perjury offense (as I understand it, that would be at the exact time he committed it today), there are other offenses they may have in mind for him, where the clock is still running.
The list of crimes that carry limitation periods longer than 5 years is extensive. Check out this report, 'Statute of Limitation in Federal Criminal Cases: An Overview' (esp pages 17-25), listing each Federal crime's limitation period.
For example, 'Deprivation of rights under color of law', a capital offense, has no limitation period.
And for crimes such as conspiracy, a 'continuing offense', the limitation clock doesn't even start until the date the accused is no longer part of the conspiracy (they need to prove it, too)