dt1 <- data.table(a = as.Date(c("2022-01-01", "2022-01-02")), b = c(1,1), t = as.difftime(1, units = "days"))
    dt2 <- data.table(a = as.Date(c("2022-01-01")), c = c(1))
    dt3 <- data.table(a = as.Date(c("2022-01-01", "2022-01-03")), c = c(1,1))

> merge(dt1, dt2, all = T)
Key: <a>
            a     b          t     c
       <Date> <num> <difftime> <num>
1: 2022-01-01     1     1 days     1
2: 2022-01-02     1     1 days    NA

> merge(dt1, dt3, all = T)
Error in rbindlist(l, use.names, fill, idcol) : 
  Class attribute on column 4 of item 2 does not match with column 4 of item 1.