The prompt product database currently has a column called expMidptMJD which is in units of seconds. These timestamps are constructed via exposure.getId().getVisitId().getDate().nsec() * 1.e-9, which means zero seconds corresponds to the daf_base DateTime nanosecond zeropoint. However, none of this is clear without some digging.
Since this field has MJD in the name, it would make sense to have MJD as units here. Another option would be to return nanoseconds and change the column name accordingly, since that is the default time unit for our daf_base DateTime objects. Either way, MJD should be a float and nanoseconds should be an int in order to have consistent behavior with DateTime.