To create a date time in the W3CDTF format (used in RSS feeds), the following @formulas could be used, and will produce something like "2003-01-21T00:03:04+01:00".
Update: John Marshall kindly gave me a fix for the formula to handle the time zones better. This is the revised formula:
t:=@Created;
y:=@Right("00" + @Text(@Year(t)); 4);
m:=@Right("0" + @Text(@Month(t)); 2);
d:=@Right("0" + @Text(@Day(t)); 2);
s1:="-";
s2:=":";
h:=@Right("0" + @Text(@Hour(t)); 2);
mm:=@Right("0" + @Text(@Minute(t)); 2);
s:=@Right("0" + @Text(@Second(t)); 2);
zone:=@Integer(0-@Zone(t));
z:=@If(zone > 0; "+"; zone < 0; "-"; "Z") +
@If(zone=0; ""; @Right("0" + @Text(zone); 2) + ":00");
y + s1 + m + s1 + d + "T" + h + s2 + mm + s2 + s + z;