Reduce granularity and remove options from a DateObject

Question

I have a DateObject with lots of granularity and additional specifications, such as the following e.g.:

dateFull = 
 DateObject[
  List[2022, 7, 18, 14, 0, 0],
  "Instant", "Gregorian", 0, "SmearedUTC",   
  List["DateTime", ":", "Millisecond"]
 ]

I would like to find a function f such that f[dateFull] returns DateObject[List[2022, 7, 18], "Day"]. This makes comparing dates easier as I can use functions such as Complement, DeleteDuplicates etc. when I am only concerned with the "Day" granularity.

In v13 I used to do this with DateObject[dateFull, "Day"] (see also DateObject granularity conversion), but in v13.1 it now returns the CalendarType ("Gregorian") as well i.e.: DateObject[List[2022, 7, 18], "Day", "Gregorian", 0].

Doing something like f = DateObject[DateString[#, {"ISODate"}], "Day"]& would work, but it feels ugly/hacky as it seems to imply that I would be better off working with Strings instead of DateObjects. (Indeed, given the ever evolving output of DateObject that may well be the case: if memory serves, DateObject[x, "Day"] behaviour was also different in v12. It seems like Wolfram is not quite happy with their dates behaviour.)

Answer 1

I got in touch with the Wolfram technical team about this and they have suggested :

timeZoneApproach=DateObject[#, "Day", TimeZone -> None]&;

FullForm[timeZoneApproach[dateFull]] (* DateObject[List[2022,7,18],"Day"] *)

The suggestion from @Jason.B is about 2 times faster on my machine, but you run the risk that the DateObject function might return the position of List and "Day" differently in future upgrades.

For completeness:

partApproach = DateObject[#, "Day"][[;; 2]] &;

pA = partApproach[dateFull]; // RepeatedTiming (*{0.0000198191,Null}*)
tZ = timeZoneApproach[dateFull]; // RepeatedTiming (*{0.0000387157,Null}*)

pA === tZ (*True*)