In recent years there appeared a need for astronomical observations timed with sub-millisecond accuracy. These include e.g. timing stellar occultations by small, sub-km or fast Near Earth Asteroids, but also tracking artificial satellites at Low Earth Orbit using optical sensors. Precise astrometry of fast-moving satellites, and accurate timing of stellar occultations have parallel needs, requiring reliable time source and good knowledge of camera delays. Thus a need for an external device that would enable equipment and camera testing, to check if they reach the required accuracy in time. We designed, constructed and thoroughly tested a New EXposure Timing Analyser (NEXTA): a GNSS-based precise timer (Global Navigation Satellite System), allowing to reach the accuracy of 0.1 millisecond, which is an order of magnitude better than in previously available tools. The device is a simple strip of blinking diodes, to be imaged with a camera under test and compare imaged time with internal camera time stamp. Our tests spanned a range of scientific cameras widely used for stellar occultations and ground-based satellite tracking. The results revealed high reliability of both NEXTA and most of the tested cameras, but also pointed that practically all cameras had internal time bias of various level. NEXTA can serve the community, being easily reproducible with inexpensive components. We provide all the necessary schemes and usage instructions.