Currently it is not possible to launch SPM SliceTiming if the input parameters slice_order and ref_slice are set in slice times (ms).

[servoz]

In fact it is only possible to use SPM SliceTiming if slice_order and ref_slice are set to slice indices.

Indeed, the SliceTimingInputSpec class of the module nipype.interfaces.spm.preprocess defines ref_slice = traits.Int(.....).
So if we set ref_slice in (ms) an execption is raised because refslice is a float and not an integer:
traits.trait_errors.TraitError: The 'ref_slice' trait of a SliceTimingInputSpec instance must be an integer, but a value of 1471.6990943396227 <class 'float'> was specified.

I propose a PR with a small correction for ref_slice (and also slice_order) so that both cases (index, or times (ms)) will work properly.

[servoz]

PR #3255 is now merged, this issue is fixed.