=SUBSTITUTE(A1,":",".")/-86400
quick explination= in cell A1 we have this(35:23) this is formated to text or general not time
so the formula is saying change this > : < for this > . < now i don't understand this myself but deviding the number by -86400 creates a time format then he goes to formating and select custom and adds this setting
s=seconds s=seconds > . < 0=milliseconds 0=milliseconds
once this is done the result will be 35.23 in time format that allows addition and sutraction