We develop a ray-tracing package for the calculation of path delays of microwave signals in the troposphere based on numerical weather models which we use for the determination of the delays of geodetic Very Long Baseline Interferometry (VLBI) observations. We show results for a two-week campaign of continuous VLBI sessions in 2008 (CONT08), where we apply those ray-traced delays and analyze the repeatability of baseline lengths in comparison to a standard approach with zenith delays and mapping functions. We find improvement in baseline length repeatabilities when no tropospheric gradients are estimated in the analysis. Furthermore, ray-traced delays are applied for Intensive sessions containing the stations Tsukuba (Japan) and Wettzell (Germany) for the determination of Universal Time (UT1). We perform an external validation using GPS-derived length-of-day values and find an improvement for UT1 with ray-traced delays by up to 4.5%.