Increment Decrement Lat long Coordinate

Hello to everyone, I have latitude and longitude coordinate. This is in degrees minutes format, I have to decrease 15 minutes in latitude coordinate and increase 15 minutes in longitude coordinate. I need the code in the vba function ex."Function Convert_Latitude(Lat As String) As Double" "Function Convert_Longitude(Long As String) As Double"
If you help me, I will be grateful to you.