# Interpolate Dates
**URL:** https://discourse.julialang.org/t/interpolate-dates/8091
**Category:** General Usage
**Tags:** dates, interpolations
**Created:** [January 1, 2018, 11:54am UTC](https://discourse.julialang.org/t/interpolate-dates/8091 "2018-01-01T11:54:47Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@anon67531922](https://discourse.julialang.org/u/anon67531922)
#### Post date: [January 1, 2018, 11:54am UTC](https://discourse.julialang.org/t/interpolate-dates/8091/1 "2018-01-01T11:54:47Z")
Similar to [this](https://discourse.julialang.org/t/linspace-with-datetime/7565), I am interested in writing an interpolation for Date(Times).
```julia
julia> using Interpolations
julia> dates = [Dates.today()+Dates.Year(i-1) for i in 1:6]
6-element Array{Date,1}:
2018-01-01
2019-01-01
2020-01-01
2021-01-01
2022-01-01
2023-01-01
julia> values = [Float64(i-1) for i in 1:6]
6-element Array{Float64,1}:
0.0
1.0
2.0
3.0
4.0
5.0
julia> int = interpolate((dates,),values,Gridded(Linear()));
julia> int[Dates.today()+Dates.Month(6)]
ERROR: MethodError: no method matching length(::Date)
Closest candidates are:
length(::SimpleVector) at essentials.jl:256
length(::Base.MethodList) at reflection.jl:558
length(::MethodTable) at reflection.jl:634
...
Stacktrace:
[1] getindex(::Interpolations.GriddedInterpolation{Float64,1,Float64,Interpolations.Gridded{Interpolations.Linear},Tuple{Array{Date,1}},0}, ::Date) at C:\Users\Eric\.julia\v0.6\Interpolations\src\gridded\indexing.jl:96
```
Any ideas? 😅
---
### Author:  [@quinnj](https://discourse.julialang.org/u/quinnj)
#### Post date: [January 3, 2018, 5:42pm UTC](https://discourse.julialang.org/t/interpolate-dates/8091/2 "2018-01-03T17:42:07Z")
Looks like it’s mainly because `length(::Date)` isn’t defined. You could do `eval(Base, :(length(::Date) = 1))` and see how far you get after that. If that’s all that’s needed, we could add that definition to `stdlib/Dates`, since I think we define that for Numbers anyway.
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [January 3, 2018, 5:45pm UTC](https://discourse.julialang.org/t/interpolate-dates/8091/3 "2018-01-03T17:45:12Z")
Numbers are iterable though. Maybe that makes a difference.
---
### Author:  [@Mastomaki](https://discourse.julialang.org/u/Mastomaki)
#### Post date: [September 3, 2021, 6:34am UTC](https://discourse.julialang.org/t/interpolate-dates/8091/4 "2021-09-03T06:34:47Z")
I am also interested in interpolation with DateTime as the x-variable.
```julia
using Interpolations, Dates
a = [DateTime(2018,1,1)+Hour(j) for j in 1:6]
println(a)
y = [3*j+1 for j in 1:6]
myinterp = LinearInterpolation(a, y)
```
ERROR: MethodError: no method matching (::Interpolations.Extrapolation{Float64,1,Interpolations.GriddedInterpolation{Float64,1,Int64,Gridded{Linear},Tuple{Array{DateTime,1}}},Gridded{Linear},Throw{Nothing}})(::DateTime)
Use square brackets for indexing an Array.
---
### Author:  [@yakir12](https://discourse.julialang.org/u/yakir12)
#### Post date: [September 3, 2021, 6:56am UTC](https://discourse.julialang.org/t/interpolate-dates/8091/5 "2021-09-03T06:56:46Z")
To solve your immidiate problem, you can:
```julia
julia> a2 = Dates.value.(a .- DateTime(2018,1,1))
6-element Vector{Int64}:
3600000
7200000
10800000
14400000
18000000
21600000
julia> myinterp = LinearInterpolation(a2, y)
```
which works.
But yeah, it would be good to either have this info somewhere visible or be able to interpolate date-times somehow.
---
### Author:  [@Mastomaki](https://discourse.julialang.org/u/Mastomaki)
#### Post date: [September 3, 2021, 7:29am UTC](https://discourse.julialang.org/t/interpolate-dates/8091/6 "2021-09-03T07:29:56Z")
Thanks!
I guess there should be a layer which maps DateTime to number if DateTime is detected.