Partially wrong. This is again dependent on the fact that there are 50 US states, thus 50 sets of rules for this.
Here in Minnesota (and in 5 or 6 other states) there is no deadline; you can register at the polling place on Election day (just takes a bit longer than pre-registered people). Also, we have the highest voter turnout in the country – not just a coincidence, in my opinion. (And I think the next highest states are also ones with same-day registration.)
In Germany this is part of the sentencing in exceptional cases (but only for two to five years). I read an article after our last federal election about how the directory of a nearby prison said he wanted an information campaign before the next election because voter turnout had been disappointing in his prison, a lot of people having failed to register the prison as their primary residence and also not having sent off for an absentee ballot.
What’s automatic is ineligibility to stand for election for five years after being sentenced to more than one year i.e. most lifers would be eligible to run for public office (but unsurprisingly no party nominates them)
In Germany, prisoners can vote.
Persons convicted to over six months of imprisonment for a crime listed in Chapter One of the Special Part of the Criminal Code (“Crimes against Peace, High Treason and Endangering the Democatic Rule of Law”) can be barred from voting for up to five years as a collateral consequence. Since this interferes with with a basic right, it can only be done by the Federal Constitutional Court.
Special Part of the Criminal Code
Section 92a Collateral Consequences
(English version)
Apart from that, you can not vote when you’re mentally incapable and have a legal guardian or if a court ordered your placement in a psychiatric hospital.
ETA: Oh, and what tschild said. 
Thank you. When this issue came up in a previous thread in the 2004 elections, I was told that there were firm cut-offs. Probably that was posted by someone in a state that had that approach, and I incorrectly generalised it.
Aren’t you saying that the UK does in fact restrict the right of convicted criminals to vote, if they are currently serving a sentence?
To clarify, their eligibility is restored one year after they have been released.
Sorry, I missed the edit window. The above is not true. Lifers having served fifteen years can be released on probation. Their electoral rights will be restored one year after their probation ended. The term of probation in this case is five years. (cite)