When I was in school training to be a Denver criminal defense lawyer, our civil procedure professor talked about statutes of limitations. He always abbreviated it as SoL, because if you didn't sue before the SoL was over, you were shit out of luck. For criminal defendants, it means you're pretty darn lucky.
A statute of limitations is basically a limit on how long the state has to prosecute crimes. In many cases, it will be meaningless, because the prosecutors usually are right on top of things. However, in other cases it goes right up there with the exclusionary rule and the very high burden of proof in criminal cases as a key tool for a Denver criminal defense lawyer.
The statutes of limitations serve several purposes. First, over a period of time the evidence for criminal cases gets worse. Witnesses forget things. Police departments dispose of evidence. Footprints get washed away, DNA decays and fingerprints fade. The idea that you could even accurately identify who committed a relatively minor crime five years ago is pretty silly. So the statute of limitations protects against inaccurate prosecution and convictions. Second, prosecutors could choose to prosecute many, many acts selectively. Without the statute of limitations, there would never be certainty as to when a case could no longer be prosecuted.
In terms of how they work, statutes of limitations are fairly complicated. In reality you'll need a Denver criminal defense lawyer to help decipher them. However, I can provide a few pointers on how they work. Generally, the statute of limitations runs from the date the crime was committed. It stops when the crime is charged. That means if the statute of limitations is two years, the prosecution has that much time to file an indictment or information. Even if the final conviction takes more than that amount of time, statute of limitations is not a defense once the crime is charged.
In certain cases, the statute can be "tolled." That means it will stop running, so it will actually go longer than the period of time prescribed in the statute. Sometimes the statute will be tolled until the prosecution finds out about the case. That's particularly the case when the defendant actively concealed the crime.
Generally, the more serious the crime, the longer the statute of limitations. In most states, murder has no statute of limitations. The limitations period for minor felonies or misdemeanors is two years or less. Other crimes are on the continuum somewhere in between. As you can see, statutes of limitations are fairly complex, so consult a lawyer if you have questions about how it might help your case.