The difference that you are getting is measured in seconds.
> head(df, n = 13)
x y fecha jul
1 1 1991-01-01 1991-01-01 1 secs
2 2 1991-01-01 1991-01-01 1 secs
3 3 1991-01-01 1991-01-01 1 secs
4 4 1991-01-01 1991-01-01 1 secs
5 5 1991-01-01 1991-01-01 1 secs
6 6 1991-01-01 1991-01-01 1 secs
7 7 1991-01-01 1991-01-01 1 secs
8 8 1991-01-01 1991-01-01 1 secs
9 9 1991-01-01 1991-01-01 1 secs
10 10 1991-01-01 1991-01-01 1 secs
11 1 1991-01-03 1991-01-03 172801 secs
12 2 1991-01-03 1991-01-03 172801 secs
13 3 1991-01-03 1991-01-03 172801 secs