R LanguageThe Date class

Remarks

Date: Stores time as number of days since UNIX epoch on 1970-01-01. with negative values for earlier dates.
It is represented as an integer (however, it is not enforced in the internal representation)
They are always printed following the rules of the current Gregorian calendar, even though the calendar was not in use a long time ago.
It doesn't keep track of timezones, so it should not be used to truncate the time out of POSIXct or POSIXlt objects.
sys.Date() returns an object of class Date

lubridate's ymd, mdy, etc. are alternatives to as.Date that also parse to Date class; see Parsing dates and datetimes from strings with lubridate.
data.table's experimental IDate class is derived from and is mostly interchangeable with Date, but is stored as integer instead of double.

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