First convert the number to a string, which you then use to create a DateTime representation of the date. These can be subtracted from each other and converted to days (datetime is the number of seconds since 01.01.1601, so to convert to days, divide by 86400).
To successfully convert to the datetime class, you have to add a delimiter between the year, month and day. Otherwise the program will interpret the first 1 of 11 to mean "January" and assume the rest is the day, leading to an error.
If you import the dates in another format the conversion might become easier.
date-conversion.fsm
