The following explains the difference between all three types of date formats:

• Standard date functions are for use with standard date formats, or just date formats. A date format refers to internally stored data that is capable of holding date components, such as century, year, quarter, month, and day. It does not include time components. A synonym does not specify an internal data type or length for a date format. Instead, it specifies display date components, such as D (day), M (month), Q (quarter), Y (2-digit year), or YY (4-digit year). For example, format MDYY is a date format that has three date components; it can be used in the USAGE attribute of a synonym. A real date value, such as March 9, 2004, described by this format is displayed as 03/09/2004, by default. Date formats can be full component and non-full component. Full component formats include all three letters, for example, D, M, and Y. JUL for Julian can also be included. All other date formats are non-full component. Some date functions require full component arguments for date fields, while others will accept full or non-full components. A date format was formerly called a smart date.
• Date-Time functions are for use with timestamps in date-time formats, also known as H formats. A timestamp value refers to internally stored data capable of holding both date and time components with an accuracy of up to a microsecond.
• Legacy date functions are for use with legacy dates only. A legacy date refers to formats with date edit options, such as I6YMD, A6MDY, I8YYMD, or A8MDYY. For example, A6MDY is a 6-byte alphanumeric string. The suffix MDY indicates the order in which the date components are stored in the field, and the prefix I or A indicates a numeric or alphanumeric form of representation. For example, a value '030599' can be assigned to a field with format A6MDY, which will be displayed as 03/05/99.

Date formats have an internal representation matching either numeric or alphanumeric format. For example, A6MDY matches alphanumeric format, YYMD and I6DMY match numeric format. When function output is a date in specified by output_format, it can be used either for assignment to another date field of this format, or it can be used for further data manipulation in the expression with data of matching formats. Assignment to another field of a different date format, will yield a random result.