Yes—this makes sense. However, I'm looking to specify that the portions of the date that I have specified are uncertain, and the portions of the date that I have not
specified are unspecified.
As such, It was unclear if 1990-uu-uu? (which would be the Level 1 format, with the uncertainty applying to the whole date) or 1990?-uu-uu (Which would be the level 2 format, where the uncertainty only applies to the specified portions of the date) was appropriate. I could be convinced that 1990?-uu-uu is appropriate, although it does not allow for, say, 199u-uu-uu or 19uu-uu-uu to be unspecified, which is unfortunate.
However, neither of these formats are supported by the BNF provided.