ok i understand thank you
(df<-structure(list(year = c(2001L, 2001L, 2001L, 2001L, 2001L, 2001L,
2001L, 2001L, 2001L, 2001L, 2001L, 2001L, 2001L, 2001L, 2001L,
2001L, 2001L, 2001L, 2001L, 2001L, 2001L, 2001L, 2001L, 2001L,
2001L, 2001L, 2001L, 2001L, 2001L, 2001L, 2001L), month = c(1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L), day = 1:31,
mtime = c("2001-01-01", "2001-01-02", "2001-01-03", "2001-01-04",
"2001-01-05", "2001-01-06", "2001-01-07", "2001-01-08", "2001-01-09",
"2001-01-10", "2001-01-11", "2001-01-12", "2001-01-13", "2001-01-14",
"2001-01-15", "2001-01-16", "2001-01-17", "2001-01-18", "2001-01-19",
"2001-01-20", "2001-01-21", "2001-01-22", "2001-01-23", "2001-01-24",
"2001-01-25", "2001-01-26", "2001-01-27", "2001-01-28", "2001-01-29",
"2001-01-30", "2001-01-31"), etp = c(0.7, 0.6, 0.7, 0.8,
0.5, 0.5, 0.5, 0.3, 0.3, 0.3, 0.3, 0.4, 0.3, 0.2, 0.3, 0.2,
0.1, 0.1, 1, 0.9, 1.4, 2.6, 5, 3, 0.6, 0.6, 0.7, 0.3, 0.3,
0.3, 0.4)), row.names = c(NA, 31L), class = "data.frame"))