Are the time stamps unchangeable after the project is completed?

Xpert-Timer has a sophisticated rights management. All functions and visibilities can be controlled via it. The right: "Add times when project is completed" can be used to regulate whether the projects are unchangeable after completion or not. So it is entirely your decision whether you want to save the times as changeable or fixed. Of course, it makes sense not to record times after the project has been completed, while adjustments before billing are common.