Recently several people have reported bugs with Klok that are severe. If you have started a timer which is currently running, do not try to change that entries start time. If you realize for example that "hey I should have started the timer 20 minutes ago" resist the urge to drag the start time. Just wait until you stop the timer and then adjust the start time. I am working on a fix for this now. An update will be posted here soon.
Sorry for the inconvenience.