From the Crystal Support web site Article ID c2001866:
Synopsis
How can a whole number, like 98268.00, be converted to a date, where 98 is the year and 268 is the number of days since January 1st?
Solution
Create a formula that formats the field as a 5 digit string, passes it to a string variable, and then parses out the first 2 digits (for the year) into a number variable.
See the formula below which uses the date function to create a January 1st date with the year number variable, and adds the number of days less 1 to that January 1st date:
In the example below, {field} is the number in the database that represents the date, i.e. 98268.
NumberVar Year1;
StringVar Date1;
Date1 :=ToText({field}, "00000"

;
Year1 :=ToNumber(Date1[1 to 2])+1900;
Date (Year1,01,01) + (ToNumber(Date1[3 to 5]) - 1);
In this particular example (field = 98268), the date returned will be September 25, 1998.