或带有anydate和excel_numeric_to_date

library(janitor)
library(anytime)
library(dplyr)
coalesce( excel_numeric_to_date(as.numeric(dat$V2)), anydate(dat$V2))
#[1] "2007-06-24" "2015-02-20" NA   

数据

dat <- structure(list(V1 = c("person_1", "person_2", "person_3"), V2 = c("39257", 
"2015/2/20", NA)), class = "data.frame", row.names = c(NA, -3L
))