Hygrothermal calculation tools have become more frequently used as an instrument in damage investigations and the moisture safety design process. Although there are tools for these purposes, there is a lack of knowledge on how to create realistic assembled calculation models that provide reliable results in the moisture safety design. During an ongoing project to create guidelines for reliable assembled calculation models to be used in the moisture safety design process, deviations linked to time shifts between measured and calculated values were observed. This paper introduces the issue and presents possible causes for deviations due to time shifts when comparing measured and calculated temperature and relative humidity. In it, a new performance metric method for evaluating hygrothermal calculation models is used to diagnose the differences. The study is an important part of the ongoing work to derive realistic levels of parameters and reasonably assembled calculation models for reliable moisture safety design. The results can also be used to make proper comparisons of measured and calculated values in damage investigations involving real climate data.