One way:

B1: =A1

Format B1 as Format/Cells/Number/Custom [s]

In article ,
Qaspec wrote:

in column a i have duration time listed for minutes and seconds as m'ss". In
column b i would like to show the same information as a but i would like to
format the value as the total seconds. Ex if a1 = 1'30" then b1=90. What is
the best way to do this?