不同输入格式下R difftime输出不同(as.character()包装器与否)

示例数据:

test <- structure(list(date1 = structure(c(1632745800, 1632745800), tzone = "UTC", class = c("POSIXct", 
"POSIXt")), date2 = structure(c(1641468180, 1641468180), tzone = "UTC", class = c("POSIXct", 
"POSIXt"))), row.names = c(NA, -2L), class = c("tbl_df", "tbl", 
"data.frame"))

为什么Difftime的输出会因为输入是否被包装成as.character而不同？例如:

library(tidyverse)

test <- structure(list(date1 = structure(c(1632745800, 1632745800), 
                                         tzone = "UTC", class = c("POSIXct", "POSIXt")), 
                       date2 = structure(c(1641468180, 1641468180), tzone = "UTC", class = c("POSIXct", "POSIXt"))), 
                  row.names = c(NA, -2L), class = c("tbl_df", "tbl", "data.frame"))

test %>% mutate(date_diff = difftime(date2, date1, units = "days"), 
date_diff2 = difftime(as.character(date2), as.character(date1), units = "days")) %>% 
  print.data.frame()
#>                 date1               date2     date_diff    date_diff2
#> 1 2021-09-27 12:30:00 2022-01-06 11:23:00 100.9535 days 100.9951 days
#> 2 2021-09-27 12:30:00 2022-01-06 11:23:00 100.9535 days 100.9951 days

在这种情况下，它只相差0.04，但有什么原因吗？哪一个会被认为是正确的？谢谢!