1. A non-transitory computer-readable recording medium having stored therein an encoding program that causes a computer to execute a process comprising: searching input text data for a date-time notation string that represents at least a date or time; generating, upon finding the date-time notation string, a converted date and time code including an identification code for identifying the date-time notation string and a normalized date-time notation formed by converting the date-time notation string into a specific date-time notation format; and converting the input text data based on conversion information that associates the converted date-time code with the date-time notation string.