Do you really mean 'UPDATE tab SET Date = nnnnn' works, but 'INSERT INTO tab (Date) VALUES (nnnnn)' doesn't? Then your dbms has a bug.
Besides it's a bad idea naming a column Date, because DATE is a reserved word in SQL. That means you have to double quote that name every time specifying it.
Like:
SELECT "Date", another_column FROM some_table
UPDATE some_table SET "Date" = nnnnn
INSERT INTO some_table ("Date", another_column) VALUES (nnnnn, yyyyy)
etc.