You have tapped the Subtract Switch. Now the Years, Months and Days, will be subtracted from the End Date and the Start Date will be set to the result: earlier from the End Date by the Number of Years, Months and Days.
Tapping on the Add Switch will reset the DaysCalc to the Add mode: the Years, Months and Days will be added to the Start Date, and the End Date will be set to the result.
The Reset Button resets the dates to the current date, Years, Months and Days to 0, and the switch to Add.
The red C Buttons reset (clear) their respective values.