Set index! not defined for WeakRefStrings. SpringArray{String,1}

General Usage
4

Okay, so that is an array of strings, so you could do something like:

col1dt = Vector{Dates.DateTime}(undef, length(col1))
for v = 1:length(col1)
    col1dt[v] = Dates.DateTime(col1[v], "d-u-y")
end

Then all your DateTime objects will be in col1dt…