Date arithmetic 

# R knows the lengths of months, e.g. that March has 31 days:
d0 <- as.Date("1968-03-01")
d0 + 31

[1] "1968-04-01"

# R also knows that 1968 was a leap year,
d1 <- as.Date("1968-02-28")
d1 + 1

[1] "1968-02-29"

# that 1900 was not a leap year,
d2 <- as.Date("1900-02-28")
d2 + 1

[1] "1900-03-01"

# that 2000 was a leap year,
d3 <- as.Date("2000-02-28")
d3 + 1

[1] "2000-02-29"

# and that leap years are 366 days long
d3 + 366

[1] "2001-02-28"