Import using metadata.opf

I am importing books by generating a metadata.opf to provide the metatdata. It works fine for author, series, tiltle, timestamp. But I can't get it to accept a published date. I have tried:-
<meta content="1988-01-01T00:00:00.000000+00:00" name="calibre:pubdate"/>

and
<meta content="1988-01-01T00:00:00.000000+00:00" name="calibre:published"/>

Any help please.

The colons after calibre in the metacontents seem to show as sticky out tongues!!!