Subtract 1 hour from the current time

Please how can I subtract 1 hour from the current our.

I have created a fieldname1 with the date turned off and the time enabled. The default time is empty so it has the current time.

I created a calculated field to subtract 1 hour from the current time.

I used this

DATETIMESUM('fieldname1', 'hh-mm', -1, 'h')

but it’s not working.

Thank you for your support.