Dates Contained in Selected Page
Looking at the pages of the preview that correspond to dated pages (ie. DiaryGridLines) we see that Q++ shows the starting and ending dates, of each page when possible, and over the course of the GridTemplate otherwise.
These pages represent exactly what Q++ will use when generating your diary output file. If these starting and ending dates look wrong to you, the generated output file is bound to have inaccuracies. To locate the source of this discrepancy, see Setting the Template Structure and [n*] : DayValue Marker Tokens.
Dates contained in the selected page are highlighted (February 9-10-11, in the example below) in the calendar to the right of the page layouts. Note that this calendar adapts itself to the weekstart property you have set (Saturday weekstart for the Muslim sample to the left below). Dates that are hidden because of the Hide Outside Days settings are also clearly identified as such (monthly sample below to the right).
Again, if you see some dates that should not be there, and vice-versa, make sure you do not have mis-typed DayValues for your tokens. If this is the case you probably also have a design-time message such as DayValue of aNumber on page of name.qxd. If you do not have any mis-typed tokens, then your grid may just be too complicated for Q++ to guess the dates it contains on its own. You then just need to help Q++ a bit by placing DayValue marker tokens.