Measuring time is not easy. It is not seen by eyes easily. Any activity we do to measure time has to be done in real time. If there is a mistake, the experiment or test has to be done again from beginning. We cannot reverse , as the time progresses forward only. When we compare time with some other standard, the activity of comparison also takes some time.. When we start measuring time, the body reaction also could delay the process and alter the measurement. So measurement of time exactly is more difficult.
Measuring length could be easier as the length can be seen, felt and measured against another visible standard length by comparison or keeping both the reference and the piece under test. Usually the length of a body does not change. It may change due to temperature. However that can be seen and measured by using instruments. Calibrating length measurement instruments is easier than calibrating time measurement instruments.