Parse a vector of numeric times, given element-wise CF-complaint time dimension units (eg. "days since 1901-01-01") #rstatstips

parse_time_dimensions.r

# https://carbon.now.sh/iYHN1JgadkF76T9rXC7q
# parse_time_dimensions: given a numeric vector of dates or datetimes, and
# a character vector of the form "[units] since [epoch]", return a POSIXct
# vector of date-times that are parsed element-wise. useful if you have,
# say, a list of netcdfs with varied epochs and units.
# example:
# > test %>% mutate(target = parse_time_dimensions(n, t))
# # A tibble: 3 x 3
#       n t                              target             
#   <int> <chr>                          <dttm>             
# 1     1 days since 1900-01-01 00:00:00 1900-01-02 00:00:00
# 2     2 years since 1920-03-14         1922-03-14 00:00:00
# 3     3 months since 1960              1960-04-01 00:00:00

parse_time_dimensions <- function(counts, dimensions) {
  
  require(tidyr)
  require(dplyr)
  require(purrr)
  require(lubridate)
  
  tibble(counts, dimensions) %>%
    separate(dimensions, into = c('units', 'epochs'), sep = ' since ') %>%
    mutate(
      epoch_dts = parse_date_time(epochs, 'ymdHMS', truncate = 5),
      units_fn = map(units,
        ~ switch(.x,
          'microseconds' = microseconds,
          'milliseconds' = milliseconds,
          'seconds' = seconds,
          'minutes' = minutes,
          'hours' = hours,
          'days' = days,
          'months' = months,
          'years' = years))) %>%
    {
      mutate(.,
        dts = pmap(
          select(., epoch_dts, units_fn, counts),
          function(epoch_dts, units_fn, counts) {
            epoch_dts + units_fn(counts)
          }))
    } %>%
    unnest(dts) %>%
    pull(dts)
}