Losing time is a psychiatric term that refers to amnesia or lost memories. This is most common with dissasociative disorders, such as Dissociative Identity Disorder.
There are many ways to lose time. The most notorious, typical of Sybil (a famous multiple from the 60s), is to "wake up" from a period of utter blackness in a strange location, having no idea what time, day, or even year it is. There can also be a kind of blurred lost time, where the memories fade out, then fade back in, rather like a dream. Often in this case, the person who lost time doesn't even realize that they have done so, because there is no sharp break in their memories. It is also possible to forget the fact that time was lost, sort of like losing time about losing time. Everyone who regularly loses time probably has different distinct ways of doing so.
For multiples, lost time occurs when a system is not co-conscious, and is very common, and can be very annoying and disconcerting. Usually the goal of therapy is to reach co-consciousness, because of the inconvenience and dangerousness of lost time.