---
title: "Dates and periods"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Dates and periods}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

`vars_to_date` accepts column names or positions. Year and month produce monthly dates; year and quarter produce quarterly dates; year, month and day produce daily dates. Year alone is also supported. Month names and quarter ranges accept English and Spanish, along with Q1, T1 and I–IV labels.

Output contains `date` as its first column. With `drop_vars = FALSE`, original component values remain unchanged. `.round = 'end'` selects the final day of the period, including leap-year February. `start` or NULL selects the beginning. `middle` means the 15th of the month, the 15th of a quarter's middle month, or July 1 for a year. Daily dates are not rounded.

A `date` column can contain Date, POSIXt, ISO dates, numeric serials, an explicit character format, or EViews periods such as 2020M02 and 2021Q04. Do not combine `date` with component arguments. Missing components, invalid periods and impossible calendar combinations raise errors.

The historical numeric origin remains 1900-01-01. For modern Excel serial dates, explicitly use `origin = '1899-12-30'`: the origin does not change automatically. Cumulative quarter labels are interpreted by their ending quarter for compatibility.

```{r}
library(Dmisc)
x <- data.frame(year = c(2020, 2021), month = c("Febrero", "February"))
result <- vars_to_date(x, year = "year", month = "month", drop_vars = FALSE)
print(result)
stopifnot(identical(result$date, as.Date(c("2020-02-29", "2021-02-28"))))
vars_to_date(data.frame(period = c("2020M02", "2021Q04")), date = "period")
```