THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
If only year, month, day are relevant to your situation, index the text string "YYYYMMDD" as Field.Keyword(String, String), rather than using DateField or Field.Keyword(String, Date). Why? Because RangeQuery will expand into a BooleanQuery of all terms in the range, and DateField represents down to the millisecond level and can expand into more terms than are allowed in a BooleanQuery.
Another reason to do this is that you will be able to index dates before the Unix Epoch (Jan 1, 1970) which DateField cannot handle.