#Data:
df1 <- as.Date(c('Jul 1', 'Jun 26', 'July 5', 'July 15'), format = '%B %d')
df2 <- c("red", "blue", "red", "blue")

df1 <- data.frame(df1,df2)

#  Group.1   x
[1]   4     red
[2]   19    blue

df <- aggregate(df1[,1], list(df1[,2]), as.numeric(max(df1[,1]) - min(df1[,1]), units="days"))

Error in match.fun(FUN) :
      'as.numeric(max(df1$date) - min(df1$date), units = "days")' is not a function, character or symbol