The data type required for my Business Document field date doesn't exist. How to define a custom one?

When you select Date & Time as the format you only get limited options for specifiying the date/time format in Flat File Business Documents:

date [YYYYMMDD]
date [YYMMDD]
date [YYYY/MM/DD]
date [YY/MM/DD]
date [DD/MM/YYYY]
date [DD/MM/YY]
customDate

For example, you would like to specifiy just time (hhmm)

-Select the customDate dataType
-Go to Attributes/Definition in the bottom panel.
-Edit the Pattern field and put the value hhmm

Data will be parsed generated following this pattern for this Business Document Field

Note the pattern is not applied in log files or you do a addTrace of the field. It will be taken into account for input and output messages.