Computing a date and time format includes obtaining a UT value of a reference time; computing intermediate data including year, month, day, hour, minute, and second, from the UT value of the reference time; computing a difference between a conversion target UT value and the UT value of the reference time using a processor; computing values of hour, minute, and second, based on the difference between the UT values; and generating a character string format representing year, month, day, hour, minute, and second, by combining the intermediate data and the values of hour, minute, and second.