It is uncommon but following coma or indeed ECT, it can happen. It often returns though. People with Catatonic schizophrenia often lose the entire period of catatonia as the memories simply have not formed. Another odd one is the dissociative fugue, where someone is functional and "wakes up" in a different town etc. This is very rare indeed.
Curious. My friend's grandfather (98!) suffered a stroke last year and has lost temporal awareness. He is very sharp in many ways and can distinguish between "this has happened" and "this is going to happen in the future". He can tell you that David Cameron is the PM at the moment, and with some prompting could probably work out more or less how long it has been since the last election. But that's the thing: he has to "work it out".
I couldn't tell you exactly when the general election was without thinking hard about it, but I know that I had a banana for breakfast this morning, and I instinctively know that the general election was much longer ago than this morning. He can't do that since the stroke. According to my friend, the consultant looking after his grandfather said that this loss of temporal awareness is very common as a result of even quite minor head trauma, but usually lasts for only a few days or weeks in younger people. As one gets older, the time it takes to recover temporal instincts gets longer.
The brain is a funny thing.