strftime converts a date-time object into a character vector. strptime does the opposite. y2000 <- as.Date("2000-01-01")
class(y2000)
# [1] "Date"
y2000_100days <- y2000 + 100
txt <- strftime(y2000_100days, '%F') # ISO 8601
class(txt)
# [1] "character"
txt
# 2000-04-10