[dheerchheda77]

grp2idx refused datetime input with "'datetime' grouping variable is not supported yet", while MATLAB accepts it. This adds support, following the existing duration path.

Changes (inst/Data_Manipulation/grp2idx.m)

• A datetime vector is accepted; a datetime matrix is refused with "'datetime' grouping variable must be a vector.", as for the other types.
• Groups are numbered by unique from datatypes, so they are in sorted order.
• NaT is treated as missing: NaN in g, and left out of gn and gl. This uses isnat in a separate branch, since isnan is not defined for datetime. The paths for the other types are unchanged.
• gn is built with cellstr, as for duration, so it follows the display format.
• Help text: lists datetime, states that datetime and duration are numbered in sorted order (duration already was, but the text said first appearance), and mentions NaT as missing.
• doc-cache regenerated for grp2idx.

Comparison with MATLAB R2026b

The new tests reproduce MATLAB's output for these cases:

Case	MATLAB R2026b and Octave
Dates with a repeat and one NaT	g = [3; 2; 3; NaN; 1], names earliest first, gl a 3x1 datetime
Custom Format	names follow the format ('2025-03-01')
Same day, different times	separate groups, names include the time
Row vector input	all outputs are columns
All NaT	g all NaN, gn 0x1 cell, gl 0x1 datetime
Empty	all outputs 0x1, gl datetime
Datetime matrix	refused
Time zone	one group, TimeZone kept on gl

Testing

• Octave 11.3.0, datatypes 1.5.0.
• test grp2idx: 65 of 65 pass. Seven tests are new and one error test was replaced.
• grpstats, crosstab and dummyvar tests still pass.

I also called every public function that uses grp2idx with a datetime grouping variable. These now work with dates: grpstats, fitdist ('By'), anova, anova1, kruskalwallis, manova1, vartestn, andrewsplot, gplotmatrix, parallelcoords, scatterhist, fitcdiscr, fitcknn, fitcnb, fitcgam, fitcnet, fitcsvm, fitcecoc, fitclinear and fitckernel. For the classifiers, predict returns datetime labels.

Follow-up work

This PR is kept to grp2idx. In the future, datetime support can be extended to all functions that call grp2idx. The remaining ones, each a small separate fix:

• Type checks that refuse datetime before calling grp2idx: crosstab (crosstab.m:79), dummyvar (dummyvar.m:179), tabulate (tabulate.m:73-76) and mnrfit (mnrfit.m:105-109).
• Classification trees: fitctree fails in treeNodeStats.m (line 93), which converts class names to text with num2str. TreeBagger and fitcensemble are refused by the 'ClassNames' type check in ClassificationTree.m (lines 874-882).