这就是sender参数的作用

private async void sDate_changed(object sender, PropertyChangedEventArgs e)
    {
        var picker = (DatePicker)sender;

        // now you can access all the properties of the picker
    }

After checking your code, I recommend you to add event DateSelected for your DatePicker.
replace code:

sDate.PropertyChanged += sDate_changed;
   fDate.PropertyChanged += fDate_changed;

with:

sDate.DateSelected += SDate_DateSelected; 
   fDate.DateSelected += FDate_DateSelected;

// here I don't have access to sdate and fdate
There are two ways to access sdate and fdate .

Method 1(just as Jason posted):

async void  SDate_DateSelected(object sender, DateChangedEventArgs e)
            {
                var picker = (DatePicker)sender;
                await DisplayAlert("Alert", "selected date is: " + picker.Date, "OK");

            }

Method 2:

Since you have defined new variables for two DatePicker ( sdate and fdate ) , you can use the two variables( sdate and fdate ) to access the selected date.
Please refer to the following code:

sDate.DateSelected += SDate_DateSelected; 
        fDate.DateSelected += FDate_DateSelected;

        //method 2
        void Recalculate()
        {
            TimeSpan timeSpan = fDate.Date - sDate.Date;

            mLabel.Text = String.Format("{0} day{1} between dates",
                                                timeSpan.Days, timeSpan.Days == 1 ? "" : "s");
        }

         async void  SDate_DateSelected(object sender, DateChangedEventArgs e)
        {
            var picker = (DatePicker)sender;
            await DisplayAlert("Alert", "SDate_DateSelected : " + picker.Date, "OK");

            //Recalculate();
        }

        async void FDate_DateSelected(object sender, DateChangedEventArgs e)
        {
            var picker = (DatePicker)sender;
            await DisplayAlert("Alert", " FDate_DateSelected : " + picker.Date, "OK");

            //Recalculate();
        }

Note:

I find that your code variable is being used incorrectly,please replace the following code:

DatePicker fDate = new DatePicker();
fDate.StyleId = field.Name;
sDate.PropertyChanged += fDate_changed;

with

DatePicker fDate = new DatePicker();
fDate.StyleId = field.Name;

// here it should be fDate not sDate
fDate.PropertyChanged += fDate_changed;