The date picker is accessed through the DatePickerSettings component and the DatePickerContent component.
use the following source to obtain access to the date picker:
var settings = GetComponent<DatePickerSettings>();
settings.Content. <you method> () to change the content
settings.Content.Selection. <your method> to modify or query the date selection
Use case example : Selecting a date range programatically
settings.Content.SelectRange(DateTime.Today,DateTime.Today +TimeSpan.FromDays(5).
Use case example : chainging the First day of the week programatically
settings.Content.FirstDayOfWeek = DayOfWeek.Monday;