Bug introduced in 10.0.2
The warning below seems unrelated to these issues: Did 10.0.2 break some forms for named slots? and 10.0.2 mapping Association to Dataset warning.
First noticed w/ TimeObject but that turns out a red herring:
data = List[Association[Rule["WEBCAM_TIME",TimeObject[List[0,12,27.9`]]],Rule["VisitStartTime",TimeObject[List[0,12,15.`]]]],Association[Rule["WEBCAM_TIME",TimeObject[List[0,12,28.`]]],Rule["VisitStartTime",TimeObject[List[0,12,15.`]]]]];
Dataset[data]

Below, named Slot lookup syntax makes no difference, eg #[x] versus #x, nor <|...|> versus Association:
Dataset[data][All,
Association[#, "time" -> #["WEBCAM_TIME"] - #["VisitStartTime"]] &]
Keys::invrl: The argument Keys[Association[Struct[{WEBCAM_TIME,Atom[String]},{Atom[TimeObject],Atom[TimeObject]}],time->-Struct[{WEBCAM_TIME,VisitStartTime},{Atom[<<1>>],Atom[<<1>>]}][VisitStartTime]+Struct[{WEBCAM_TIME,VisitStartTime},{Atom[TimeObject],Atom[TimeObject]}][WEBCAM_TIME]]] is not a valid Association or a list of rules. >>
and similarly for Values::invrl, even though "time" is handled correctly:

Finally, casting TimeObject to AbsoluteTime makes no diff either, eg:
Dataset[data][All, All, AbsoluteTime][All,
Association[#, "time" -> #["WEBCAM_TIME"] - #["VisitStartTime"]] &]

Gives a similar warning but refers to Atom[Real] and Atom[Integer] as per AbsoluteTime.
Is this another type inference fault?
Datasetbecause of its type system complications?TitanicandPlanetsare toy models. – alancalvitti Dec 23 '14 at 15:29Datasetregressions in 10.0.2 and will be revising our testing of it in the future. – Stefan R Dec 23 '14 at 21:29