Problem overview

In R, calling as.POSIXlt("1994-01-01") sometimes produces the error:

"character string is not in a standard unambiguous format"

Meanwhile, as.POSIXlt("1993-12-31") and as.POSIXlt("1994-01-02") work fine. Developers wondering why a single date fails discovered this happens in certain time zones. For example, in the America/Lima time zone, there was a one‑hour clock change at midnight on 1 January 1994: clocks jumped from 23:59:59 on 31 December 1993 directly to 01:00:00 on 1 January 1994. In other words, midnight on that day never existed, so R can’t parse it.

Why this happens

R’s as.POSIXlt assumes midnight (00:00:00) when you supply a date without a time. If your system’s default time zone is set to America/Lima, the date “1994-01-01” corresponds to a non‑existent local time because the clocks jumped forward one hour at 00:00. R therefore throws an error since the string is ambiguous. Other dates before and after are valid because those midnights existed.

Solution

There are two ways to resolve this error:

1. Specify a time zone where midnight existed: Passing a tz argument forces R to interpret the date in a specific time zone. For example:

as.POSIXlt("1994-01-01", tz = "UTC")
# [1] "1994-01-01 UTC"

UTC has no daylight‑saving shifts on that date, so the date parses normally. You can also choose any other time zone without a gap at midnight.

2. Convert to a Date and then to POSIX: If you don’t care about time zones, convert to a Date and then to POSIX. Using as.Date() avoids time‑zone interpretation. When you later need POSIX, supply an explicit time zone:

d <- as.Date("1994-01-01")
# Convert to POSIX with a safe timezone
as.POSIXlt(d, tz = "UTC")

This two‑step approach bypasses the ambiguous midnight altogether.

Conclusion

The ambiguous-date error occurs because R cannot represent a date at a time that never existed in the local time zone. Specifying a different tz or converting via as.Date are simple ways to avoid the error. Always be mindful of daylight‑saving or historical time‑zone changes when working with dates and times