1) yearmon/yearqtr Create a monthly sequence using yearmon class and then convert that to the start and end dates. Similarly for quarters and yearqtr class. Internally both represent dates by year and fraction of year so use 1/12 and 1/4 in by=. Also note that using as.Date gives the date at the start of the month or quarter and the same but with the frac=1 argument gives the end.
library(zoo)
# input
st <- as.Date("1990-01-01")
en <- as.Date("2021-12-01")
# by month
mon <- seq(as.yearmon(st), as.yearmon(en), 1/12)
paste(as.Date(mon), as.Date(mon, frac = 1), sep = ":")
# by quarter
qtr <- seq(as.yearqtr(st), as.yearqtr(en), 1/4)
paste(as.Date(qtr), as.Date(qtr, frac = 1), sep = ":")
There is some question of what the end date should be. The above give an end date on the last interval of 2021-12-31 but if the end date should be 2021-12-01 so that no interval extends past en then replace the two paste lines with these respectively.
paste(as.Date(mon), pmin(as.Date(mon, frac = 1), en), sep = ":")
paste(as.Date(qtr), pmin(as.Date(qtr, frac = 1), en), sep = ":")
2) Base R A base R alternative is to use the expressions involving cut shown below to get the end of period. (1) seems less tricky but this might be useful if using only base R were desired. A similar approach with pmin as in (1) could be used if we want to ensure that no range extends beyond en.
This and the remaining solutions, but not (1), assume that st is the first of the month; however, that could readily be handled if needed.
mon <- seq(st, en, by = "month")
paste(mon, as.Date(cut(mon + 31, "month")) - 1, sep = ":")
qtr <- seq(st, en, by = "quarter")
paste(as.Date(qtr), as.Date(cut(qtr + 93, "month")) - 1, sep = ":")
3) lubridate Using various functions from this package we can write the following. A similar approach using pmin as in (1) could be used if the ranges may not extend beyond en.
library(lubridate)
mon <- seq(st, en, by = "month")
paste(mon, mon + month(1) - 1, sep = ":")
qtr <- seq(st, en, by = "quarter")
paste(qtr, qtr + quarter(1) - 1, sep = ":")
4) IDate We can use IDate class from data.table in which case we can make use of cut.IDate which returns another IDate object rather than a character string (as in base R).
st <- as.IDate("1990-01-01")
en <- as.IDate("2021-12-01")
mon <- seq(st, en, by = "month")
paste(mon, cut(mon + 31, "month") - 1, sep = ":")
qtr <- seq(st, en, by = "quarter")
paste(qtr, cut(qtr + 93, "month") - 1, sep = ":")
