Here is a way to separate the strings and convert them to Date and difftime values so they can be used in calculations.
library(stringr)
library(dplyr)
#>
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#>
#> filter, lag
#> The following objects are masked from 'package:base':
#>
#> intersect, setdiff, setequal, union
library(lubridate)
#>
#> Attaching package: 'lubridate'
#> The following objects are masked from 'package:base':
#>
#> date, intersect, setdiff, union
library(hms)
#>
#> Attaching package: 'hms'
#> The following object is masked from 'package:lubridate':
#>
#> hms
DF <- data.frame(DateTime=c("12/30/1899 0:03:43","12/31/1899 11:03:43"))
DF
#> DateTime
#> 1 12/30/1899 0:03:43
#> 2 12/31/1899 11:03:43
DF2 <- DF |> mutate(Date=mdy(str_extract(DateTime,"../../..")),
Time=as_hms(str_extract(DateTime,"(?<= ).+$")))
str(DF2)
#> 'data.frame': 2 obs. of 3 variables:
#> $ DateTime: chr "12/30/1899 0:03:43" "12/31/1899 11:03:43"
#> $ Date : Date, format: "2018-12-30" "2018-12-31"
#> $ Time : 'hms' num 00:03:43 11:03:43
#> ..- attr(*, "units")= chr "secs"
Created on 2021-12-26 by the reprex package (v2.0.1)