Assuming you have a date field for both, you can make a formula field to combine the inputs like so:
IF(AND({date Fom}, {Date To}), DATETIME_FORMAT({Date From}, 'YYYY/MM/DD') & ' - ' & DATETIME_FORMAT({Date To}, 'YYYY/MM/DD'))
The formula asks “Have both dates been filled in? If so, write the date like 2020/02/09 - 2020/12/01”