The ‘as.Date’ Time Zone Anomaly

The core issue lies in how ‘as.Date’ interprets date strings when a time zone offset is present. If the system's time zone has a positive offset from GMT (e.g., GMT+5:30 for India Standard Time), ‘as.Date’ may implicitly convert the input date string to UTC before discarding the time component. This conversion can cause a date to effectively "fall back" into the previous day if the time component of the original string (implicitly midnight) is before the offset.

Behavior on Linux Systems

On Linux, the ‘TZ’ (time zone) environment variable plays a crucial role in how ‘as.Date’ behaves.

‘TZ’ Not Set

When the ‘TZ’ environment variable is not explicitly set, ‘as.Date’ will shift the date back one day if the system time zone is GMT+x.

# set TZ=""  # i.e. not set
Sys.getenv("TZ")
#> [1] ""
as.Date("2025-02-01")
#> [1] "2025-01-31"
as.POSIXct("2025-02-01")
#> [1] "2025-02-01 GMT+5:30"

'TZ' Set to GMT

Setting 'TZ' to "GMT" ensures that 'as.Date' interprets the date string in the GMT time zone, thus preventing the date shift.

# set TZ=GMT  # via .TERRenviron
Sys.getenv("TZ")
#> [1] "GMT"
as.Date("2025-02-01")
#> [1] "2025-02-01"
as.POSIXct("2025-02-01")
#> [1] "2025-02-01 GMT"

'TZ' Set to IST

Similarly, setting 'TZ' to "IST" (or any other specific time zone) resolves the issue on Linux.

# set TZ=IST  # via .TERRenviron
Sys.getenv("TZ")
#> [1] "IST"
as.Date("2025-02-01")
#> [1] "2025-02-01"
as.POSIXct("2025-02-01")
#> [1] "2025-02-01 GMT+5:30"

Behavior on Windows Systems

Unlike Linux, setting the 'TZ' environment variable on Windows 10 does not consistently resolve the 'as.Date' shifting issue.

'TZ' Not Set

Even when 'TZ' is not set, 'as.Date' still shifts the date back.

# System TimeZone: UTC+5:30
 
# set TZ=""  # i.e. not set
Sys.getenv("TZ")
#> [1] ""
(da <- as.Date("2025-02-01"))
#> [1] "2025-01-31"
dput(da)
#> structure(19023, class = "Date")
as.POSIXct("2025-02-01")
#> [1] "2025-02-01 GMT+5:30"

'TZ' Set to GMT

Setting 'TZ' to "GMT" on Windows does not prevent the date shift for 'as.Date'.

# set TZ=GMT  # via .TERRenviron
Sys.getenv("TZ")
#> [1] "GMT"
(da <- as.Date("2025-02-01"))
#> [1] "2025-01-31"
dput(da)
#> structure(19023, class = "Date")
as.POSIXct("2025-02-01")
#> [1] "2025-02-01 GMT"

'TZ' Set to IST

Similar to GMT, setting 'TZ' to "IST" on Windows does not resolve the 'as.Date' issue.

# set TZ=IST  # via .TERRenviron
Sys.getenv("TZ")
#> [1] "IST"
(da <- as.Date("2025-02-01"))
#> [1] "2025-01-31"
dput(da)
#> structure(19023, class = "Date")
as.POSIXct("2025-02-01")
#> [1] "2025-02-01 GMT+5:30"

All

A robust workaround that functions consistently across both Linux and Windows is to explicitly specify the 'format' argument in the 'as.Date' call, ensuring that no wildcard characters are used within the format string. 

# Explicitly set the exact format (no wild card characters) e.g.

as.Date("2025-02-01", format="%Y-%m-%d")  # works
#> [1] "2025-02-01"

as.Date("2025-02-01", format="%Y.%m.%d")  # fails, with . wild card
#> [1] "2025-01-31"

As demonstrated, providing the exact format string ('"%Y-%m-%d"') resolves the date shift.

However, using a format string with wildcards or inexact matches (like '"%Y.%m.%d"') can still lead to the problem.

Conclusion

The 'as.Date' function's behavior with time zones, particularly with positive GMT offsets, can lead to subtle date shifts. While setting the 'TZ' environment variable can provide a solution on Linux, Windows users will likely need to rely on the more explicit 'format' argument in their 'as.Date' calls to ensure correct date interpretation. Always specifying the exact format when converting strings to dates is a good practice to avoid unexpected results related to time zone differences.

When working with dates in Spotfire Enterprise Runtime for R (TERR), specifically with the ‘as.Date’ function, users might encounter unexpected behavior where dates appear to shift by a day. This typically occurs when the system's time zone is set to GMT+x hours, where 'x' is a positive value. This article will explore this phenomenon across different operating systems and offer potential workarounds.