Change the column type from date to minutes (only minutes and seconds are display)

Question

I wanna change my column from date to decimal numbers (but I want display minutes and seconds like that : MM.SS). My column looks like this:

I wrote the code but as a result I got an error.

XYZ$Start <- if(format(XYZ$Start, format="%H") = 1) {
  as.numeric(format(XYZ$Start, format="%M.%S"))+60
} else {
  as.numeric(format(XYZ$Start, format="%M.%S"))
}