Preserve CF time coordinate in read_netcdf

CHANGELOG.md

@@ -115,6 +115,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- **`read_netcdf` threw on files with a CF time coordinate.** A time axis
+  written by `write_netcdf(...; time=t)` (units `"seconds since ..."`, a
+  `calendar`) is decoded by NCDatasets to `DateTime`, which the reader tried to
+  store in a `Dict{String, Vector{Float64}}` — a `convert` error. The
+  `"coordinates"`/`"variables"` containers are now `Any`-valued, so the decoded
+  time is preserved (`data["coordinates"]["time"]::Vector{DateTime}`) alongside
+  the `Float64` spatial coordinates.
+
 - **Unstructured RL/RLZ evaluation flipped every sine (odd-in-λ) component.**
   `_eval_unstructured_rl`/`_eval_unstructured_rlz` (behind `evaluate_unstructured`,
   `interpolate_to_grid`, and grid relocation) synthesized the Fourier series with

src/io.jl

@@ -1092,18 +1092,26 @@ and return its contents as a plain [`Dict`].
 
 The returned dictionary has four string-keyed entries:
 
-| Key            | Type                          | Contents                          |
-|:-------------- |:----------------------------- |:--------------------------------- |
-| `"dimensions"` | `Dict{String, Int}`           | dimension name → length           |
-| `"coordinates"`| `Dict{String, Vector{Float64}}`| coordinate name → values         |
-| `"variables"`  | `Dict{String, Array{Float64}}`| data variable name → array        |
-| `"attributes"` | `Dict{String, Any}`           | global attribute name → value     |
+| Key            | Type                | Contents                          |
+|:-------------- |:------------------- |:--------------------------------- |
+| `"dimensions"` | `Dict{String, Int}` | dimension name → length           |
+| `"coordinates"`| `Dict{String, Any}` | coordinate name → values          |
+| `"variables"`  | `Dict{String, Any}` | data variable name → array        |
+| `"attributes"` | `Dict{String, Any}` | global attribute name → value     |
 
 Coordinate variables are identified by sharing their name with a NetCDF
 dimension.  All other variables are returned under `"variables"`.  This
 format-independent structure can be passed directly to plotting libraries
 or used for further analysis without requiring a [`SpringsteelGrid`](@ref).
 
+Values are returned exactly as NCDatasets decodes them: spatial coordinates and
+data come back as `Float64` arrays, while a CF time axis (a coordinate carrying
+`units = "... since ..."` and a `calendar`) is decoded to a `DateTime` (or
+`CFTime` for non-standard calendars) array. The `"coordinates"`/`"variables"`
+containers are therefore `Any`-valued so the decoded time is preserved rather
+than forced into `Float64` — reading a file written with `write_netcdf(...;
+time=t)` returns `data["coordinates"]["time"]::Vector{DateTime}`.
+
 # Arguments
 - `filename`: Path to an existing NetCDF file.
 
@@ -1133,17 +1141,20 @@ function read_netcdf(filename::String)
         end
         result["dimensions"] = dims
 
-        # Coordinates (variables that share their dimension name)
-        coords = Dict{String, Vector{Float64}}()
+        # Coordinates (variables that share their dimension name). Any-valued:
+        # NCDatasets decodes a CF time axis to DateTime/CFTime, which must not be
+        # coerced to Float64 — preserving it is the whole point of a time axis.
+        coords = Dict{String, Any}()
         for name in keys(dims)
             if haskey(ds, name)
                 coords[name] = Array(ds[name])
             end
         end
         result["coordinates"] = coords
 
-        # Variables (everything that's not a coordinate)
-        vars = Dict{String, Array{Float64}}()
+        # Variables (everything that's not a coordinate). Any-valued for the same
+        # reason as coordinates (a data variable may also carry a decoded type).
+        vars = Dict{String, Any}()
         for name in keys(ds)
             if !haskey(dims, name)
                 vars[name] = Array(ds[name])

test/io.jl

@@ -850,6 +850,38 @@ using DataFrames
                 @test haskey(data["variables"], "u_xx")
             end
 
+            @testset "read_netcdf preserves CF time coordinate" begin
+                # write_netcdf(...; time=t) writes a CF time axis (units
+                # "seconds since 1970-...", calendar gregorian), which NCDatasets
+                # decodes to DateTime. read_netcdf must preserve it, not coerce to
+                # Float64. Regression: it previously threw a convert error.
+                gp = SpringsteelGridParameters(geometry="R", num_cells=20,
+                    iMin=0.0, iMax=10.0,
+                    vars=Dict("u" => 1),
+                    BCL=Dict("u" => CubicBSpline.PERIODIC),
+                    BCR=Dict("u" => CubicBSpline.PERIODIC))
+                grid = createGrid(gp)
+                pts = getGridpoints(grid)
+                for i in eachindex(pts)
+                    grid.physical[i, 1, 1] = sin(2π * pts[i] / 10.0)
+                end
+                spectralTransform!(grid)
+
+                tmpfile = joinpath(mktempdir(), "test_r_time.nc")
+                write_netcdf(tmpfile, grid; time=3600.0)
+
+                data = read_netcdf(tmpfile)   # must not throw
+
+                @test haskey(data["coordinates"], "time")
+                t_vals = data["coordinates"]["time"]
+                @test eltype(t_vals) <: Dates.AbstractTime
+                # 3600 s since the 1970 epoch = 1970-01-01T01:00:00
+                @test t_vals[1] == DateTime(1970, 1, 1, 1, 0, 0)
+                # spatial coordinate still decodes to Float64
+                @test data["coordinates"]["x"] isa AbstractVector{Float64}
+                @test haskey(data["variables"], "u")
+            end
+
             @testset "read_netcdf nonexistent file throws" begin
                 @test_throws Exception read_netcdf("/nonexistent/path/file.nc")
             end