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Summary

Adds Series.diffDate and Series.diffDateOffset functions that compute the TimeSpan difference between consecutive DateTime / DateTimeOffset values in a series — addressing the feature request in #565.

Motivation

The existing Series.diff uses an F# inline type-constrained implementation requiring homogeneous subtraction (T - T = T). DateTime - DateTime = TimeSpan is heterogeneous, so Series.diff cannot be used for timestamp series and would raise a runtime type error if attempted.

Implementation

The new functions use shift + zipInto internally:

let diffDate offset (series:Series<'K, DateTime>) : Series<'K, TimeSpan> =
    (series, series |> shift offset) ||> zipInto (fun curr prev -> curr - prev)
```

This produces exactly the same semantics as numeric `diff`:

```
result[k] = series[k] - series[k - offset]

For offset = 1, this gives the elapsed time since the previous observation — useful for detecting periodicity, irregular intervals, gaps, etc.

New API surface

Usage example

// Detect time intervals between observations
let timestamps =
    series [ 1 => DateTime(2024,1,1,0,0,0)
             2 => DateTime(2024,1,1,1,0,0)   // +1h
             3 => DateTime(2024,1,1,1,30,0)  // +30m
             4 => DateTime(2024,1,1,3,0,0) ] // +1.5h

timestamps |> Series.diffDate 1
// Series(int, TimeSpan):
//   2 -> 01:00:00
//   3 -> 00:30:00
//   4 -> 01:30:00

Trade-offs

• Adds two new exported functions; naming (diffDate / diffDateOffset) mirrors the existing diff naming convention.
• Implementation is slightly less efficient than the low-level index-shift path used by numeric diff, but the difference is negligible for typical use cases and the clarity is worth it.

Test Status

Build: ✅ dotnet build src/Deedle/Deedle.fsproj -c Release — succeeded, 0 errors.

Tests: ✅ 4 new tests added in tests/Deedle.Tests/Series.fs; all pass. Full suite: 467 passing (1 pre-existing failure in "Can expand properties based on runtime type information" — unrelated to this PR).

Note: dotnet fake build has an infrastructure issue in this environment (FAKE TypeInitializationException); the dotnet build / dotnet test paths were used instead.

Closes #565

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