[Exim] Multiple Date: fields

Nice easy one, I hope. It's just that my eyes glaze over when reading
syntax specifications in RFCs, and I'd like confirmation of my
interpretation before moaning to someone (_not_ about Exim)

Question: is it legal for a message to contain multiple Date: headers?

My answer: No, according to both RFC 822 and RFC2822.

Reason for asking:

I'm being sent a message with multiple Date: headers.
Exim concatenates them, resulting in a long $header_date:
Nigel's system filter then fails it as overlong (to foil a M$ buffer
overflow exploit)

I'm quite happy about what Exim is doing; I just want right on my side
when I complain to the sender ...

TIA,
Richard Hall