# DataTables : convert Nullable{String}("14:00:00") to Dates.Minute
**URL:** https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543
**Category:** Data
**Tags:** question, dates, convert
**Created:** [March 8, 2017, 3:29pm UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543 "2017-03-08T15:29:55Z")
**Posts on this page:** 14
**Page:** 1
### Author:  [@Fred](https://discourse.julialang.org/u/Fred)
#### Post date: [March 8, 2017, 3:29pm UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/1 "2017-03-08T15:29:55Z")
Hi,
I have a DataTable with two columns of time and I want to compute the time duration between these 2 columns. The problem I have is to compute a duration with Nullable type.
```julia
julia> a, b
(Nullable{String}("14:00:00"),Nullable{String}("15:15:00"))
julia> b-a
ERROR: MethodError: no method matching -(::String, ::String)
in -(::Nullable{String}, ::Nullable{String}) at /home/fred/.julia/v0.5/NullableArrays/src/operators.jl:132
julia> convert(Dates.Minute,a)
ERROR: MethodError: Cannot `convert` an object of type Nullable{String} to an object of type Base.Dates.Minute
This may have arisen from a call to the constructor Base.Dates.Minute(...),
since type constructors fall back to convert methods.
julia> Dates.DateTime(a,"HH:MM")
ERROR: MethodError: Cannot `convert` an object of type Nullable{String} to an object of type Int64
This may have arisen from a call to the constructor Int64(...),
since type constructors fall back to convert methods.
in DateTime(::Nullable{String}, ::String) at ./dates/types.jl:116
```
Thanks for your help !
---
### Author:  [@quinnj](https://discourse.julialang.org/u/quinnj)
#### Post date: [March 8, 2017, 3:40pm UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/2 "2017-03-08T15:40:22Z")
> [@Fred](#):
>
> a, b
> (Nullable{String}(“14:00:00”),Nullable{String}(“15:15:00”))
You probably want something like:
```julia
a, b = (Nullable{String}("14:00:00"),Nullable{String}("15:15:00"))
Base.Dates.Time(str::String) = Dates.Time(map(x->parse(Int, x), split(str, ':'))...)
difflift(a, b) = (isnull(a) || isnull(b)) ? Nullable{Dates.Nanosecond}() : Nullable{Dates.Nanosecond}(b - a)
aT, bT = Dates.Time(get(a)), Dates.Time(get(b))
difflift(aT, bT)
```
The idea here is you’re doing “lifting” on the nullable arguments in case either is actually null. Also, this is currently using 0.6 code where the `Dates.Time` type has been added. You could also use a DateTime without too much trouble since you’re just looking for the difference.
---
### Author:  [@Fred](https://discourse.julialang.org/u/Fred)
#### Post date: [March 8, 2017, 3:49pm UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/3 "2017-03-08T15:49:57Z")
@quinnj Thank you very much for your answer. I did not imagine it was so complex to compute a time difference 😲
I am currently using Julia 5.0 and I have this result :
```julia
julia> aT, bT = DateTime(get(a)), DateTime(get(b))
ERROR: ArgumentError: Delimiter mismatch. Couldn't find first delimiter, "-", in date string
in parse(::String, ::Base.Dates.DateFormat) at ./dates/io.jl:169
in DateTime(::String) at ./dates/io.jl:268
```
So the solution is only possible with Julia 0.6 ?
---
### Author:  [@nalimilan](https://discourse.julialang.org/u/nalimilan)
#### Post date: [March 8, 2017, 4:34pm UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/4 "2017-03-08T16:34:28Z")
On Julia 0.6, the `.(` and `.-` operators will automatically lift arguments for you, so this will be enough:
```julia
julia> a, b = (Nullable{String}("14:00:00"),Nullable{String}("15:15:00"))
(Nullable{String}("14:00:00"), Nullable{String}("15:15:00"))
julia> Base.Dates.Time(str::String) = Dates.Time(map(x->parse(Int, x), split(str, ':'))...)
julia> a2 = Dates.Time.(a)
Nullable{Base.Dates.Time}(14:00:00)
julia> b2 = Dates.Time.(b)
Nullable{Base.Dates.Time}(15:15:00)
julia> b2 .- a2
Nullable{Base.Dates.Nanosecond}(4500000000000 nanoseconds)
```
@quinnj Looks like we need parsing methods to create `Time` objects from strings?
---
### Author:  [@cormullion](https://discourse.julialang.org/u/cormullion)
#### Post date: [March 8, 2017, 4:49pm UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/5 "2017-03-08T16:49:36Z")
On Julia v0.5:
```julia
julia> Dates.DateTime(get(b), "HH:MM:SS") - Dates.DateTime(get(a), "HH:MM:SS")
4500000 milliseconds
```
although if you look the date is Jan 1st 1 AD/CE, which is a bit distracting… 🙂
---
### Author:  [@Fred](https://discourse.julialang.org/u/Fred)
#### Post date: [March 8, 2017, 5:02pm UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/6 "2017-03-08T17:02:16Z")
Thank you !
@nalimilan
I tried on julia 0.6 and I still have a problem to convert nanoseconds to minutes for example
```julia
julia> Base.Dates.Time(str::String) = Dates.Time(map(x->parse(Int, x), split(str, ':'))...)
julia> a2 = Dates.Time.(a)
Nullable{Base.Dates.Time}(14:00:00)
julia> b2 = Dates.Time.(b)
Nullable{Base.Dates.Time}(15:15:00)
julia> b2 .- a2
Nullable{Base.Dates.Nanosecond}(4500000000000 nanoseconds)
julia> Int(b2 .- a2)
ERROR: MethodError: Cannot `convert` an object of type Nullable{Base.Dates.Nanosecond} to an object of type Int64
This may have arisen from a call to the constructor Int64(...),
since type constructors fall back to convert methods.
Stacktrace:
[1] Int64(::Nullable{Base.Dates.Nanosecond}) at ./sysimg.jl:24
julia> c=b2 .- a2
Nullable{Base.Dates.Nanosecond}(4500000000000 nanoseconds)
julia> c/1e3
ERROR: MethodError: no method matching /(::Nullable{Base.Dates.Nanosecond}, ::Float64)
Closest candidates are:
/(::Float64, ::Float64) at float.jl:377
/(::BigFloat, ::Union{Float16, Float32, Float64}) at mpfr.jl:343
/(::Complex, ::Real) at complex.jl:268
...
```
with the solution of @cormullion it finaly works better on Julia 0.5 because all operations on times are possible
```julia
julia> a, b = (Nullable{String}("14:00:00"),Nullable{String}("15:15:00"))
(Nullable{String}("14:00:00"),Nullable{String}("15:15:00"))
julia> (Nullable{String}("14:00:00"), Nullable{String}("15:15:00"))
(Nullable{String}("14:00:00"),Nullable{String}("15:15:00"))
julia> Dates.DateTime(get(b), "HH:MM:SS") - Dates.DateTime(get(a), "HH:MM:SS")
4500000 milliseconds
julia> c = Dates.DateTime(get(b), "HH:MM:SS") - Dates.DateTime(get(a), "HH:MM:SS")
4500000 milliseconds
julia> c*60
270000000 milliseconds
julia> Int(c)/1e3
4500.0
julia> Int(c)/1e3/60
75.0
```
---
### Author:  [@nalimilan](https://discourse.julialang.org/u/nalimilan)
#### Post date: [March 8, 2017, 5:12pm UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/7 "2017-03-08T17:12:06Z")
Everything which works in Julia 0.5 also works on 0.6. The question is, do you want to extract the value from the `Nullable` wrapper immediately (which implies that no null values are possible): then use `get` first. OTOH, if you need to support missing values, then keep the `Nullable`, but then you need to use `.(`, `.-`, etc. for all operations.
---
### Author:  [@Fred](https://discourse.julialang.org/u/Fred)
#### Post date: [March 9, 2017, 8:22am UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/8 "2017-03-09T08:22:55Z")
Thank you very much @nalimilan ! I understand better the role of the get function.
Since, to my knowledge, this useful function does not appear in the DataTables documentation, a short explanation how to extract the value from the Nullable wrapper would be very useful in the section “The Nullable Type”.
```julia
julia> c = b2 .- a2
Nullable{Base.Dates.Nanosecond}(4500000000000 nanoseconds)
julia> d = get(c)
4500000000000 nanoseconds
julia> Int(d)
4500000000000
julia> Int(d)/1e9
4500.0
```
---
### Author:  [@nalimilan](https://discourse.julialang.org/u/nalimilan)
#### Post date: [March 9, 2017, 10:01am UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/9 "2017-03-09T10:01:14Z")
Yeah, that section of the DataTables manual could be improved. It should also point to the relevant section of the Julia manual. Would you make a pull request to change this?
---
### Author:  [@Fred](https://discourse.julialang.org/u/Fred)
#### Post date: [March 9, 2017, 10:50am UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/10 "2017-03-09T10:50:23Z")
@nalimilan I tried to make a pull request (for the first time 🙂) and I did not succeed. I did not find the “documentation” section of DataTables.jl on GitHub.
Do I have to fork the whole JuliaData/DataTables.jl directory to make a pull request ? Then on the “Compare changes” I don’t know what to choose. Sorry for asking those questions, but it is unclear of that works 😳
---
### Author:  [@nalimilan](https://discourse.julialang.org/u/nalimilan)
#### Post date: [March 9, 2017, 11:23am UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/11 "2017-03-09T11:23:26Z")
It’s here: [https://github.com/JuliaData/DataTables.jl/blob/master/docs/src/man/getting\_started.md](https://github.com/JuliaData/DataTables.jl/blob/master/docs/src/man/getting_started.md) You can click on the small pen button to open a pull request directly from GitHub.
---
### Author:  [@Fred](https://discourse.julialang.org/u/Fred)
#### Post date: [March 9, 2017, 11:35am UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/12 "2017-03-09T11:35:35Z")
Thank you @nalimilan, I did it, I hope it is correct 😉
---
### Author:  [@nalimilan](https://discourse.julialang.org/u/nalimilan)
#### Post date: [March 10, 2017, 12:37pm UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/13 "2017-03-10T12:37:12Z")
Sorry, but I can’t see it…
---
### Author:  [@Fred](https://discourse.julialang.org/u/Fred)
#### Post date: [March 11, 2017, 10:05am UTC](https://discourse.julialang.org/t/datatables-convert-nullable-string-1400-to-dates-minute/2543/14 "2017-03-11T10:05:32Z")
@nalimilan I forgot to validate, now it appears 🙂