I am trying to figure out how to calculate turnaround time in excel.

For example:
Sample 1 - submitted on 01/05/2018 (cell A1)

Test 1 - Completed on 01/07/2018 (Cell B1)
Test 2 - Completed on 01/15/2018 (Cell C1)
Test 3 - Completed on 01/08/2018 (Cell D1)

I want to calculate the overall number of days taken from sample submission date to last test date, however i need the formula to automatically capture the latest date between cells B1 to D1.