Dates Data

structure(list(date = structure(1:10, .Label = c("2009-05-01", 
"2009-05-15", "2009-06-01", "2009-06-15", "2009-10-01", "2009-10-15", 
"2010-01-01", "2010-01-15", "2010-07-01", "2010-07-15"), class = "factor"), 
    coeff = c(2, 3, 2.5, 4, 3.65, 4.8, 2.4, 1.9, 5.2, 4.3)), .Names = c("date", 
"coeff"), row.names = c(NA, -10L), class = "data.frame")
         date coeff
1  2009-05-01  2.00
2  2009-05-15  3.00
3  2009-06-01  2.50
4  2009-06-15  4.00
5  2009-10-01  3.65
6  2009-10-15  4.80
7  2010-01-01  2.40
8  2010-01-15  1.90
9  2010-07-01  5.20
10 2010-07-15  4.30