Should pre UNIX epoch fractional dates be supported?

[TimTaylor]

Currently this does not seem to work

(x <- .Date(0) - 1.5)
#> [1] "1969-12-30"

# correct
as.POSIXlt(x)$mday
#> [1] 30

# wrong
ymd::mday(x)
#> [1] 31

^{Created on 2025-04-24 with reprex v2.1.1}