ISOdate conversion issues

I am trying to convert dates with ISOdate function, but I face 2 issues:

• converting a date before 1970 gives an NA;
• converting a date after 1970 offsets by 1 month.

Here is the code I use:

firstDate <- as.Date("1910-01-01", "%Y-%m-%d")
todayDate <-  Sys.Date() #Lets say its 2021-09-04
firstDatePOSIX <- as.POSIXlt(firstDate)
todayDatePOSIX <- as.POSIXlt(todayDate)
firstDateISO <- ISOdate(firstDatePOSIX$year + 1900, firstDatePOSIX$mon, firstDatePOSIX$mday)
todayDateISO <- ISOdate(todayDatePOSIX$year + 1900, todayDatePOSIX$mon, todayDatePOSIX$mday)

firstDateISO returns NA

todayDateISO returns "2021-08-04 12:00:00 GMT" instead of "2021-09-04 12:00:00 GMT"

My questions:

• What to do to get "1910-01-01 12:00:00 GMT" with firstDateISO?
• What to do to get "2021-09-04 12:00:00 GMT" with todayDateISO?

Any help much appreciated.

System used:

• R version: 4.1.1 (2021-08-10)
• RStudio version: 1.4.1717
• OS: macOS Big Sur version 11.5.1

Use mon + 1.

with(firstDatePOSIX, ISOdate(year + 1900, mon + 1, mday))
# [1] "1910-01-01 12:00:00 GMT"

with(todayDatePOSIX, ISOdate(year + 1900, mon + 1, mday))
# [1] "2021-09-04 12:00:00 GMT"