Function fdtm uses System.DateTime.ToString(String) method, which converts the value of this instance to its equivalent string representation using the specified format. 

Syntax is fdtm(variable,format_string), where format_string is a constant or report variable with format pattern. For detail format description see ".NET Framework Developer's Guide" section "Custom DateTime Format Strings" 

Example: 

If you have a DateTime variable date1 which is January 15, 2007, time is set to 10:30 

and format_var='"The meeting is on" dddd, MMMM d, yyyy, "at" hh:mm' 

Report field \fdtm(date1,format_var)\ in the report result will be: The meeting is on Monday, January 15, 2007, at 10:30