Transit events of extrasolar planets off er a wealth of information for planetary characterization. However, for many known targets, the uncertainty of their predicted transit windows prohibits an accurate scheduling of follow-up observations. In this work, we refine the ephemerides of 21 hot Jupiter exoplanetswith the largest timing uncertainties. We collected 120 professional and amateur transit light curves of the targets of interest, observed with a range oftelescopes of 0.3 m-2.2 m, and analyzed them along with the timing information of the planets discovery papers. In the case of WASP-117b, we measured a timing deviation compared to the known ephemeris of about 3.5 h, and for HAT-P-29b and HAT-P-31b the deviation amounted to about 2 h and more. For all targets, the new ephemeris predicts transit timings with uncertainties of less than 6 min in the year 2018 and less than 13 min until 2025. Thus, our results allow for an accurate scheduling of follow-up observations in the next decade.