As a workaround, you could change your LetterDate variable's format to numeric and store the dates in ISO format (ie YYYYMMDD). You could then use field coding to ensure the stored number is output in whatever date format you require, using a field coded as:
{QUOTE"{=MOD(MOD(DOCVARIABLE LetterDate,10000),100)}-{=INT(MOD(DOCVARIABLE LetterDate,10000)/100)}-{=INT(DOCVARIABLE LetterDate/10000)}" \@ "DDDD, DD MMMM YYYY"}
With this approach, you can change the date switch around to whatever day, month & year order and format you prefer.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]