减去POSIXct时如何在相同单位中产生时差

I want to subtract to POSIXct. I can do this but depending on the first row (i guess?) the difference will be in seconds or minutes. Below you can see the first diff is in seconds and the second diff is in minutes because I changed the time difference in the first row:I want to subtract to POSIXct. I can do this bu