Set DateTimePicker element via AutomationElement

You can get the native window handle and send DTM_SETSYSTEMTIME message to set the selected date for DateTimePicker control.

To do so, I suppose you have found the element, then you can use follwing code:

var date =  new DateTime(1998, 1, 1);
DateTimePickerHelper.SetDate((IntPtr)element.Current.NativeWindowHandle, date);

DateTimePickerHelper

Here is the source code for DateTimePickerHelper. The class has a public static SetDate method which allow you to set a date for the date time picker control:

using System;
using System.Runtime.InteropServices;
public class DateTimePickerHelper {
    const int GDT_VALID = 0;
    const int DTM_SETSYSTEMTIME = (0x1000 + 2);
    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
    struct SYSTEMTIME {
        public short wYear;
        public short wMonth;
        public short wDayOfWeek;
        public short wDay;
        public short wHour;
        public short wMinute;
        public short wSecond;
        public short wMilliseconds;
    }
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    static extern IntPtr SendMessage(IntPtr hWnd, int msg, 
        int wParam, SYSTEMTIME lParam);
    public static void SetDate(IntPtr handle, DateTime date) {
        var value = new SYSTEMTIME() {
            wYear = (short)date.Year,
            wMonth = (short)date.Month,
            wDayOfWeek = (short)date.DayOfWeek,
            wDay = (short)date.Day,
            wHour = (short)date.Hour,
            wMinute = (short)date.Minute,
            wSecond = (short)date.Second,
            wMilliseconds = 0
        };
        SendMessage(handle, DTM_SETSYSTEMTIME, 0, value);
    }
}