# Dates.DateTime format in a DataFrame

**URL:** <https://discourse.julialang.org/t/dates-datetime-format-in-a-dataframe/57935>\
**Category:** Data\
**Tags:** dates, csv, time\
**Created:** [March 25, 2021, 2:09pm UTC](https://discourse.julialang.org/t/dates-datetime-format-in-a-dataframe/57935 "2021-03-25T14:09:27Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Laco\_Kovac](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laco_kovac/32/20684_2.png) [@Laco\_Kovac](https://discourse.julialang.org/u/Laco_Kovac)\
**Post date:** [March 25, 2021, 2:09pm UTC](https://discourse.julialang.org/t/dates-datetime-format-in-a-dataframe/57935/1 "2021-03-25T14:09:27Z")

</div>

I am fetching content of the table and downloading it CSV (using LibPQ, DataFrames and CSV).  
This is the code converting DataFrame to CSV:

```julia
executeResult = execute(conn, "SELECT * FROM table")
frame = DataFrame(executeResult)
join(CSV.RowWriter(frame))

```

And this is roughly the result:

```julia
date,gascons,irradiation,rain,temperature,windspeed,ph,_label
2004-01-01T00:00:00.0,53875.51,0.0,0.5,0.0,0.0,0.0,0.0
2004-01-01T01:00:00.0,51297.24,0.0,0.3,0.8,5.5,0.0,0.0
2004-01-01T02:00:00.0,51010.4,0.0,0.5,0.6,6.3,0.0,0.0

```

The problem is that “T” in the column with type Dates.DateTime. The resulting CSV should look like this:

```julia
date,gascons,irradiation,rain,temperature,windspeed,ph,_label
2004-01-01 00:00:00.0,53875.51,0.0,0.5,0.0,0.0,0.0,0.0
2004-01-01 01:00:00.0,51297.24,0.0,0.3,0.8,5.5,0.0,0.0
2004-01-01 02:00:00.0,51010.4,0.0,0.5,0.6,6.3,0.0,0.0

```

I achieve it easily, using `replace(join(CSV.RowWriter(frame)), "T" => " ")` instead of the last line in the code above, but this would work for me only because all other columns are numeric. I also think this is not an efficient solution esp. for large input table (huge string with many T’s to replace). Could I somehow ask DataFrame to supply Dates.DateTime column in a different format?

Thank you.

---

<div class="post-metadata">

**Author:** ![quinnj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/quinnj/32/11_2.png) [@quinnj](https://discourse.julialang.org/u/quinnj)\
**Post date:** [March 25, 2021, 2:34pm UTC](https://discourse.julialang.org/t/dates-datetime-format-in-a-dataframe/57935/2 "2021-03-25T14:34:10Z")

</div>

If you’re already materializing the query result as a DataFrame, you should just use `CSV.write` instead of `CSV.RowWriter`. The latter is for when you have a true row-streaming case where you don’t want to materialize the full intermediate table in memory (like creating a DataFrame).

Something like this should work:

```julia
transform(col, val) = val
transform(col, val::DateTime) = Dates.format(val, dateformat"yyyy-mm-dd HH:MM:SS.s")
CSV.write("output.csv", frame; transform=transform)

```

---

<div class="post-metadata">

**Author:** ![Laco\_Kovac](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laco_kovac/32/20684_2.png) [@Laco\_Kovac](https://discourse.julialang.org/u/Laco_Kovac)\
**Post date:** [March 25, 2021, 4:12pm UTC](https://discourse.julialang.org/t/dates-datetime-format-in-a-dataframe/57935/3 "2021-03-25T16:12:26Z")

</div>

I cannot use CSV.write because I don’t want to write in CSV file, just to hand the data in the string to another service.

---

<div class="post-metadata">

**Author:** ![Skoffer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/skoffer/32/378_2.png) [@Skoffer](https://discourse.julialang.org/u/Skoffer)\
**Post date:** [March 25, 2021, 4:23pm UTC](https://discourse.julialang.org/t/dates-datetime-format-in-a-dataframe/57935/4 "2021-03-25T16:23:21Z")

</div>

You still can use `CSV.write` since it accepts `IOBuffer`

```julia
julia> df = DataFrame("A" => [1, 2, 3], "B" => ["A", "B", "C"], "T" => DateTime.([0, 1, 2
]))
3×3 DataFrame
 Row │ A B T
     │ Int64 String DateTime
─────┼────────────────────────────────────
   1 │ 1 A 0000-01-01T00:00:00
   2 │ 2 B 0001-01-01T00:00:00
   3 │ 3 C 0002-01-01T00:00:00

julia> transform(col, val) = val
julia> transform(col, val::DateTime) = Dates.format(val, dateformat"yyyy-mm-dd HH:MM:SS.s")
julia> iob = IOBuffer()

julia> CSV.write(iob, df, transform = transform)
julia> String(take!(iob))
"A,B,T\n1,A,0000-01-01 00:00:00.0\n2,B,0001-01-01 00:00:00.0\n3,C,0002-01-01 00:00:00.0\n
"

```

---

<div class="post-metadata">

**Author:** ![Laco\_Kovac](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laco_kovac/32/20684_2.png) [@Laco\_Kovac](https://discourse.julialang.org/u/Laco_Kovac)\
**Post date:** [March 26, 2021, 7:49am UTC](https://discourse.julialang.org/t/dates-datetime-format-in-a-dataframe/57935/5 "2021-03-26T07:49:34Z")

</div>

It works like a charm. Thank you!