The article touches upon the issues of determining the time of commission of a crime as one of the facts to be proven. Basing on the analysis of the law in force and the compliance practices of statement of the time of commission of a crime a conclusion has been made that total uncertainty is not admissible while referring the time of commission of a crime: to use the wording „at an unknown time“, „within the period until… (the investigation has not determined the precise time)“. It has been brought to notice that, it is possible for the investigation, basing on the evidence assembled, to state in the prosecution the time of a crime commission by defining specified time („at 12:30, 12 September 2018“), days („October 7, 2017“), month and year („in September 2018“), the season of the year („In spring 2017“), or any time interval („in period from 13 p.m. to 18 o’clock 12 September 2018“). It has been noted, that precision in determining the time of commission of a crime depends on the remoteness of the event under investigation and the amount of evidence that has been assembled. Mistakes have been considered, introduced while determining the time of commission of a crime. Proposals have been formulated in the matter of improving the investigator’s activity intended to determine the time of commission of a crime.