Inspect or change TIMEVAL.
There can only be one time variable.
Examples
# Set TIMEVAL
x1 <-
px(population_gl) |>
px_timeval('year')
# Print TIMEVAL
px_timeval(x1)
#> [1] "year"
# Remove TIMEVAL
x2 <- px_timeval(x1, NULL)
px_timeval(x2)
#> NULL