Neither the science data nor the housekeeping data coming from the instrument contains actual date and time stamps. Only the On-Board Time in units of 2^-16 ticks are available and stored in the telemetry packets. In order to enable the matching of these tick-times to actual dates a special reference file will be present on each FOT containing an OBT to UTC conversion table. This file is used to calculate a reference time (MJDREF) for each new observation. To obtain the reference date the instrument time is matched with the latest OBT that is preceeding the instrument time (ticks) so that the reference can be calculated as follows: if SCSEQBEG contains the ticks of the first event and OBT is the closest time in the OBT_UTC files, then MJDREF = UTC - OBT / ( 2^-16 * 86400) where UTC is the time corresponding to the OBT value The following problems are provided for: a. In case the time is falling outside the provided conversion OBTs a warning is issued. Then depending which is closest to the instrumemnt time either the first or the last OBT is taken to calculate the reference data (MJDREF). b. The OBT counter may scroll though zero during the observation. This will happen every 2^16 (65535) seconds or 18.2 hours. This can be easily solved as during one Observing Period the starttimes within one observation must not occur before the endtimes of previous observations. Only the first one will take the earliest OBT match.