The Brandenburg v. Ohio (1969) Supreme Court case allows for criminalizing speech only if the speech is "directed to inciting or producing imminent lawless action and is likely to incite or produce such action" [1]. "imminent" means that there has to be a near-future, clear time window. "I will kill X president within 3 days" could be illegal. "I will kill X president within a year" is too vague. Regardless, either one could be interpreted as evidence of criminal intent to harm the president. (If you were only joking about killing the president and the jury believes you, then you're fine.)

[1] https://en.wikipedia.org/wiki/Brandenburg_v._Ohio