Someone declares that the changes are aimed at combating untimeliness and incompleteness in [name of tax] tax assessment and withholding by tax agents.
How would you express the said idea without mentioning those who declare that? There is no precise information on authorities or officials who stated the purposes of these changes.
I am not sure whether the 'declare' in the passive voice works here?
The changes are declared to be aimed at [...].
It's awkward here - not because of grammar, but because of word choice. "Declare" is a strong word. It's emphatic and formal. Because of that, it's unusual - even suspicious - for the person (entity) making the declaration to be unknown or unnamed.
Because of that, a passive construction makes it seem that something is missing, and the reader is left to wonder about it. Does the writer know who made the declaration, but isn't divulging the information? Or does the writer happen to not know? Or is it something that no one knows?
I would recommend using an active construction, saying something like, "An unnamed source declares..." but even then, I would strongly consider a synonym like "states."
I am not sure whether the 'declare' in the passive voice works here?
It works, but it is not ideal. I would use the past tense, since you are referring to a specific event (the declaration):
It was declared that the changes are aimed at...
The changes are declared to be..
This sets it up as a premise, or axiom.. But this is what you are doing when using an unnamed source anyway. So, the buck stops with this sentence, to some degree.
A verb needs a subject,
"It" will suffice - it is (has been) declared...
Alternatively use declaration as the subject - the declaration states...
Of course you can use the passive voice -- but I'm sorry the entire construction doesn't sound like idiomatic English to me. I work in a government tax-related office in North America. This isn't the sort of thing that's "declared" (and to boot, "untimeliness" and "incompleteness" are awkward, even for government-speak.
Why not try something like "the stated aim of the changes is to discourage [combat] untimeliness and incompleteness in assessment [of the tax]..." or better yet, "...to encourage timely and complete assessment..." "Stated aim" is neutral as to source of statement/declaration of intent -- and is also agnostic as to the sincerity of the claimed purpose.
