Translation:The date of my request is March fifteenth.
Note that the hints are generally not context-sensitive to the particular question, and often list words that don't work. That said, I think "claim" is a valid translation for "demander" here.
The word apparently just isn't on Duo's list of accepted answers for this question. You can report it and maybe they will add it.
Numbers are one of the last things that someone translates in their head. I.e. if you're reasonably fluent in French and there are numbers interspersed in the text, you'll read the numbers in English, not French. As such, it's appropriate to not allow using digits when translating and insist on words.
As for not using "of", although I am not British I am familiar with British English and it seems much more normal to me to include "of". Do you have a source that says "of" can be dropped? A quick search on Google ngrams shows that "fifteenth March" is used about 1% of "fifteenth of March", indicating that dropping "of" is a mistake.
My concern is not around using the digit versus word, duolingo will normally accept "15th" instead of "fifteenth" when translating from the language you are learning to the language you know.
My issue was, when using the digit and "th" superscript in British English we would not write of. If i was writing out in words i would use the "fifteenth of March".
i.e. "The date of my request is the 15th March." would be fine in English. As would "The date of my request is March fifteenth." or "... the fifteenth of March" but not "fifteenth March"
Yes, i understand but that is why they have oral tests on Duolingo as well as written ones. It should not be teaching you to write "of" when it is not normal to do so in written English.
It is similar to silent letters (of which there are many in French); you are expected to write them but not to say them; in this case it is reversed, you are expected to say them but not to write them.
Surely true comprehension of the language is getting used to this finer detail.
If you're learning English and you think the correct phrase is "15th March", i.e. not spoken as "the fifteenth of March", then you're wrong. Duolingo has no way of distinguishing between foreign learners and native speakers. As a native speaker, you are in a better position to accept that Duolingo will mark it wrong, than a foreign learner having their misconception marked right.
"The 15th of March asks for me"? I really don't know what you're trying to say. "The 15th of March every year is a difficult day for me"? Is that what you mean? I really don't see how how a date can be demanding, except in a really figurative sense. Maybe it could be (you'd need to check this) « Le quinze mars est exigeant à moi ».
I think you're getting confused with the somewhat false-friend nature of « demander ». In English "to demand" is very forceful, but it's much weaker/more polite in French (to request). If you have a really strong need to translate this sentence, I'd suggest asking your question on http://french.stackexchange.com/
The Duo folks have to hand enter every possible translation. They start with US English, and then add UK English if they think of it or it is reported to them.
Interestingly, in the US we would say "March fifteenth" or "the fifteenth of March", but generally not "fifteenth March" (which has a rather UK sound to me).