`fwrite(dateTimeAs = "write.csv")` is not aligned with `write.csv()`

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start by reproducing the minimal example with fwrite(dateTimeAs = "write.csv"), write.csv(), and options("digits.secs"). Then locate the existing fwrite dateTimeAs coverage and determine how timestamp precision is selected; done means fwrite preserves the millisecond portion by default in the same way as write.csv, with regression coverage.

Written by the indexing model from the issue text.

Description

I just tried to replace some write.csv calls with fwrite using its dateTimeAs = "write.csv" option and noticed that by default (without setting digits.secs) fwrite removes the millisecond portion from the timestamps, which is not (or no longer) the case when using write.csv.

The background was discussed here. However, it seems only a corresponding test was adapted and fwrite was not aligned with write.csv.

Is this intentional? I think it's confusing.

Minimal reproducible example

library(data.table)

options("digits.secs")
# options(digits.secs = 3L)

DT <- data.table(TS = as.POSIXct("2026-02-26 13:21:49.123"))

write.csv(DT, file = "write.csv", quote = FALSE, row.names = FALSE)
readLines("write.csv")
# [1] "TS" "2026-02-26 13:21:49.123"

fwrite(DT, file = "fwrite.csv", dateTimeAs = "write.csv")
readLines("fwrite.csv")
# [1] "TS" "2026-02-26 13:21:49"

sessionInfo()

R version 4.5.2 (2025-10-31 ucrt)
Platform: x86_64-w64-mingw32/x64
Running under: Windows 10 x64 (build 19045)

Matrix products: default
  LAPACK version 3.12.1

locale:
[1] LC_COLLATE=English_United States.utf8  LC_CTYPE=English_United States.utf8   
[3] LC_MONETARY=English_United States.utf8 LC_NUMERIC=C                          
[5] LC_TIME=English_United States.utf8    

time zone: Europe/Berlin
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.18.2.1

loaded via a namespace (and not attached):
[1] compiler_4.5.2    cli_3.6.5         tools_4.5.2       rstudioapi_0.18.0 pkgload_1.5.0    
[6] rlang_1.1.7
Dominant language
R
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