Question about time coordinate reading in .nc file with time unit nanoseconds from start Date Time

[willcoxe]

Hi! Thanks for making the package!

I have several NetCDF files that I am trying to read in and I am running into an issue with the time coordinate. This may be related to CFTime issue 18 because the file I am reading in has "nanoseconds since" as the time unit. However the error I get is not the same as the one I get from CFTime.

Having nanosecond precision is not super important to me at the moment, but I am not sure how to circumvent this to be able to run ds["time"][:] without error. Apologies if this is somewhere in the docs and I simply have not found it. I appreciate any insight/advice you might have.

fn = "file.nc"
ds = NCDataset(fn, "r")

julia> ds["time"]
time (2588)
  Datatype:    Int64
  Dimensions:  time
  Attributes:
   units                = nanoseconds since 2022-08-16 20:02:30.000998258
   calendar             = proleptic_gregorian

julia> ds["time"].var[:]
2588-element Vector{Int64}:
                 0
     5400000001669
                 ⋮
 13926600000002623
 13932000000000715

julia> ds["U_mag"][:]
2588×39 Matrix{Union{Missing, Float32}}:
 ⋮      ⋱  

julia> ds["time"][:]
ERROR: MethodError: Cannot `convert` an object of type Int64 to an object of type DateTime

Closest candidates are:
  convert(::Type{DateTime}, ::Millisecond)
   @ Dates /usr/share/julia/stdlib/v1.10/Dates/src/conversions.jl:34
  convert(::Type{DateTime}, ::Date)
   @ Dates /usr/share/julia/stdlib/v1.10/Dates/src/conversions.jl:30
  convert(::Type{DateTime}, ::T2) where T2<:Union{DateTimeJulian, DateTimeProlepticGregorian, DateTimeStandard}
   @ CFTime ~/.julia/packages/CFTime/n09Um/src/CFTime.jl:308
  ...

Stacktrace:
 [1] setindex!(A::Vector{DateTime}, x::Int64, i1::Int64)
   @ Base ./array.jl:1021
 [2] macro expansion
   @ ~/.julia/packages/NCDatasets/c8XyT/src/cfvariable.jl:538 [inlined]
 [3] macro expansion
   @ ./simdloop.jl:77 [inlined]
 [4] CFtransformdata
   @ ~/.julia/packages/NCDatasets/c8XyT/src/cfvariable.jl:537 [inlined]
 [5] getindex(v::NCDatasets.CFVariable{…}, indexes::Colon)
   @ NCDatasets ~/.julia/packages/NCDatasets/c8XyT/src/cfvariable.jl:604
 [6] top-level scope
   @ REPL[164]:1
Some type information was truncated. Use `show(err)` to see complete types.

julia> CFTime.timedecode(ds["time"].var[:],"nanoseconds since 2022-08-16 20:02:30.000998258")
ERROR: unknown units nanoseconds

OS: Arch linux julia version 1.10.1

[Alexander-Barth]

Here is an example. Does it work for you?

t0  = DateTime(2022,08,16,20,02,30,001)  # or parse from ds["time"].attrib["units"]
t = t0 .+ Nanosecond.(ds["time"].var[:])

In order to parse the time origin, you might want to have a look here:

https://github.com/JuliaGeo/CFTime.jl/blob/382cfd5918c8c47b2a9c8306cf73b41a405f1735/src/CFTime.jl#L361

[willcoxe]

Apologies for not replying earlier. I have been using the DateTime package with the Nanoseconds conversion successfully. Thanks!