A Serna motion is a motion to dismiss for violation of an accused’s right to a speedy trial. It gets its name from the California Supreme Court case Serna v. Superior Court (1975) 40 Cal.3d 239.

While the statute of limitations is calculated from the time of the incident until the case is charged, what happens if after the case is charged, nothing happens for longer than the statute of limitations would have allowed without the accused being brought to court? This is what Serna decided.

The California Supreme Court discussed how to determine post-accusation delay in its benchmark case, Serna v. Superior Court (1985) 40 Cal.3d 239:

“Petitioner here contends that the four-and-one-half-year delay in arresting him following the filing of the misdemeanor complaint is presumptively prejudicial and thus the trial court must assume some inherent prejudice and engage in this weighing process, putting the People to the burden of justifying the delay by showing that legitimate law enforcement concerns caused or contributed to the delay.

We agree that the delay here was presumptively prejudicial. The length of the delay between the filing of the complaint and the arrest of defendant far exceeded the one-year limitation period applicable to misdemeanors generally. Had there been no complaint on file this prosecution would have been statutorily barred. Statutes of limitation reflect a legislative construction of the speedy trial guarantee. ( Barker v. Municipal Court (1966) 64 Cal.2d 806, 812 [51 Cal. Rptr. 921, 415 P.2d 809].) Although the period of limitation for some misdemeanors which might have been charged as felonies is now three years (see § 805), the one-year period of limitation for ‘any misdemeanor’ was part of the Penal Code on its adoption in 1872. (See former § 801.) A court may appropriately conclude that delays between the filing of a complaint and the arrest of a defendant which exceeds the typical one-year period of limitation generally applicable to misdemeanors are unreasonable and thus presumptively prejudicial within the contemplation of the speedy trial guarantee. ‘[The] concept of a period of limitation developed in recognition of the ever increasing difficulty faced by both the government and a criminal defendant in obtaining reliable evidence (or any evidence at all) as time passes following the commission of a crime . . . . [para. ] Other policy considerations which underlie the concept of a period of limitation vary in purpose. The possibility of self-reformation by the criminal offender may lessen the need for society to impose corrective sanctions and society’s impulse for retribution may correspondingly diminish as time passes. . . . Finally, adoption of a period of limitation represents a legislative recognition that for all but the most serious of offenses (such as murder or kidnaping) a never-ending threat of prosecution is more detrimental to the functioning of a civilized society than it is  beneficial.’ ( People v. Zamora (1976) 18 Cal.3d 538, 546-547 [134 Cal. Rptr. 784, 557 P.2d 75].) It would be anomalous in light of the congruent objectives of the speedy trial guarantee and the legislatively adopted one-year period of limitation that has governed misdemeanors for over a century if, after a decision has been made to prosecute an offense as a misdemeanor, the mere filing of a misdemeanor complaint without further action by the state for a period in excess of one year were not presumed to be a violation of the right to a speedy trial, and the People compelled to justify the delay.

Our conclusion that a delay between the filing of a misdemeanor complaint and the arrest and prosecution of a defendant which exceeds one year is unreasonable and presumptively prejudicial is virtually compelled by prior decisions of this court. In Harris v. Municipal Court (1930) 209 Cal. 55 [285 P. 699], we noted that section 1382, which required dismissal of a felony information if the defendant had not been brought to trial within 60 days unless good cause for the delay was shown was a legislative determination that a trial after that unjustified delay was not a speedy trial, and we extended to misdemeanants the same reasoning even in the absence of a statutory requirement of dismissal. In that case, in which the delay between the filing of the complaint and the arrest of the defendant was 18 months, we stated: ‘It would be most unreasonable to hold that a delay of eighteen months in the trial of the action was not in violation of the constitutional right to a speedy trial when . . . the petitioner was at all times available for the service of process and that the delay was not at all traceable to him.’ (209 Cal. at pp. 62-63.) In a companion misdemeanor case, Gutterman v. Municipal Court (1930) 209 Cal. 65 [285 P. 703], again the complaint had been filed some 18 months prior to the arrest of the defendant, and again this court ordered dismissal because the People failed to show good cause for the delay. In each case the delay was considered unreasonable and thus prejudice was presumed with dismissal being constitutionally compelled in the absence of a demonstration of good cause for the delay.

The measuring period utilized by the court in Harris and Gutterman was the 60-day period mandated by the Legislature for felony prosecutions. The Legislature has since amended section 1382, which now reflects a legislative determination that dismissal is not required unless there is a delay in excess of 45 days between arraignment and trial or 30 days if the defendant is in custody. Although this manifests a legislative conclusion that the delay between the filing of the complaint and arraignment should not be considered in determining if a delay in bringing a misdemeanor defendant to trial is unreasonable, the one-year period of the generally applicable misdemeanor statute of limitations remains as a touchstone for measuring the reasonableness of a delay between complaint and arrest. If a delay of one year in charging a misdemeanor defendant is so unreasonable that prosecution is statutorily barred, it follows that a delay of similar duration must be considered unreasonable and presumptively prejudicial within the contemplation of the Sixth Amendment when, although a complaint has been filed, the defendant is not arrested and arraigned on the complaint for that period.

Therefore, although there may be cases in which a defendant is able to demonstrate actual prejudice from delays of shorter duration, he need not do so when the delay exceeds one year. Delays of that magnitude are presumptively prejudicial.”

(Serna v. Superior Court (1985) 40 Cal.3d 239, 252-254 [219 Cal.Rptr. 420, 707 P.2d 793] [bold added].)