Hmmm
.
I must say I don’t understand the problem with Unique()
here either 
(which is bothering me a lot
)
And there’s another thing I don’t understand concerning the formulas :
Why in both the results the very first date is formatted as YYYY/MM/DD
but the remaining results are formatted as MM/D/YYYY
? 

This makes no sense 
I’ve taken another look at your doc anyway (because it got me thinking
) and came up with something but I’m not sure if this is correct
.
I’ve reproduced your table and added a field Name
because working on this, I couldn’t differentiate the rows using solely the dates.
I think I’m on the right path, but profoundly unsure 
First, I’ve filtered by dates creating a field Filter Date
with the formula :
(Getting all the rows for the date)
Filter([Unique dates],RezervationDate=thisRow.RezervationDate)
Then I’ve filtered that field by ID
(in the field Filter Date ID
)
(Creating some kind of “cumulative sum”)
Filter(thisRow.[Filter Date],ID<=thisRow.ID).Unique()
And then, in the field Date ID = Date
, I’ve compared both of those fields to extract the unique values for the dates, getting rid of the duplicates (not sure I can say this differently
) with the formula :
Filter(thisRow.[Filter Date],thisRow.[Filter Date ID]=thisRow.[Filter Date])
And finally, I get to the Rooms
and the Sum()
(The formulas on the canva are just below the table
)