# Interpolate DateTime without InexactError

**URL:** <https://discourse.julialang.org/t/interpolate-datetime-without-inexacterror/31033>\
**Category:** General Usage\
**Created:** [November 13, 2019, 9:42am UTC](https://discourse.julialang.org/t/interpolate-datetime-without-inexacterror/31033 "2019-11-13T09:42:55Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![scelles](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/scelles/32/220408_2.png) [@scelles](https://discourse.julialang.org/u/scelles)\
**Post date:** [November 13, 2019, 9:42am UTC](https://discourse.julialang.org/t/interpolate-datetime-without-inexacterror/31033/1 "2019-11-13T09:42:55Z")

</div>

Hello,

I need to interpolate `DateTime`

I have a function like

```julia
function interpolate_time(x1::Float64, t1::DateTime, x2::Float64, t2::DateTime, xI::Float64)
    return (xI - x1) / (x2 - x1) * (t2 - t1) + t1
end

```

```julia
interpolate_time(0.0, DateTime(2019, 1, 1), 2.0, DateTime(2019, 1, 1, 0, 0, 40), 1.0)
2019-01-01T00:00:20

```

works fine but

```julia
> interpolate_time(0.0, DateTime(2019, 1, 1), 2.0, DateTime(2019, 1, 1, 0, 0, 40), 1.5)

```

raises an `InexactError`

```julia
ERROR: InexactError: Int64(1.5)
Stacktrace:
 [1] Type at ./float.jl:703 [inlined]
 [2] * at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.1/Dates/src/periods.jl:92 [inlined]
 [3] * at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.1/Dates/src/periods.jl:93 [inlined]
 [4] interpolate_time(::Float64, ::DateTime, ::Float64, ::DateTime, ::Float64) at ./REPL[9]:2
 [5] top-level scope at none:0

```

How to perform such an interpolation (accepting a 1 millisecond error)?

I noticed that there is a package named Interpolations.jl (pinging @tim.holy )

[https://github.com/JuliaMath/Interpolations.jl](https://github.com/JuliaMath/Interpolations.jl)

but I don’t know if it can handle that and/or if that’s not overkilled for that purpose (it’s a simple linear interpolation here)

Kind regards

---

<div class="post-metadata">

**Author:** ![anowacki](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/anowacki/32/17375_2.png) [@anowacki](https://discourse.julialang.org/u/anowacki)\
**Post date:** [November 13, 2019, 11:01am UTC](https://discourse.julialang.org/t/interpolate-datetime-without-inexacterror/31033/2 "2019-11-13T11:01:14Z")

</div>

This should fit the bill:

```julia
using Dates
function interpolate_time(x1, t1::DateTime, x2, t2::DateTime, xI)
    Δ = Dates.value(t2 - t1)
    Millisecond(round(Int, Δ*(xI - x1)/(x2 - x1))) + t1
end

```

```julia
julia> interpolate_time(0, DateTime(2019, 1, 1), 2, DateTime(2019, 1, 1, 0, 0, 40), 1.5)
2019-01-01T00:00:30

```

Other options than `round` would be `floor` or `ceil` depending on whether you want to always round to the nearest, lowest or highest `Int` respectively.

---

<div class="post-metadata">

**Author:** ![scelles](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/scelles/32/220408_2.png) [@scelles](https://discourse.julialang.org/u/scelles)\
**Post date:** [November 13, 2019, 11:12am UTC](https://discourse.julialang.org/t/interpolate-datetime-without-inexacterror/31033/3 "2019-11-13T11:12:56Z")

</div>

Thanks @anowacki for your help.

I wonder what is the highest interval between 2 times which is allowed by such a function.

`xI` beeing between `x1` and `x2`, `(xI - x1)/(x2 - x1)` is between `0` and `1`

So I did

```julia
> typemax(Millisecond)
9223372036854775807 milliseconds

```

to have an idea… but maybe we can have a more human readable value?

---

<div class="post-metadata">

**Author:** ![anowacki](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/anowacki/32/17375_2.png) [@anowacki](https://discourse.julialang.org/u/anowacki)\
**Post date:** [November 13, 2019, 11:19am UTC](https://discourse.julialang.org/t/interpolate-datetime-without-inexacterror/31033/4 "2019-11-13T11:19:17Z")

</div>

Let’s find out the date from the year 0 (if it existed!) when `typemax(Millisecond)` ms have elapsed:

```julia
julia> DateTime(0) + typemax(Millisecond)
292277024-08-17T07:12:55.807

```

That’s about 292 million years.

---

<div class="post-metadata">

**Author:** ![scelles](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/scelles/32/220408_2.png) [@scelles](https://discourse.julialang.org/u/scelles)\
**Post date:** [November 13, 2019, 11:28am UTC](https://discourse.julialang.org/t/interpolate-datetime-without-inexacterror/31033/5 "2019-11-13T11:28:46Z")

</div>

simple and efficient. I won’t be there to see if Julia is still useful. Thanks again.