During the past decades, Ocean Bottom Seismometers (OBS) have played a key role in permanent seismic activity monitoring at sea as well as allowing a better understating of the earth interior. Data collected by the instrument can provide information on the ocean bottom sub-layers down to a depth of 40 km beneath the ocean floor. The accuracy of the results directly depends on the time drift of the equipment due to change of environmental conditions. Time base of the OBS is given by a unique stable crystal oscillator. This paper presents the time drift study of an Ocean Bottom Seismometer in real environmental conditions. By means of a climate chamber, temperature and humidity tests of a general purpose time base generator were carried out and crystal temperature stability and time drift were calculated. Furthermore, the behaviour of the time drift of the instrument has been evaluated in order to correct the data in the data processing stage.