时间序列：从单个列中的多个类别的初始值中减去后续值

I have a df like this one:

我有一个像这样的df：

cat<-c("a","a","a","b","b","b","c","c","c")
julian_day<-c(100,112,123,192,207,237,34,87,102)
dat<-data.frame(cat,julian_day)
cat<