Thread:Help talk:Timeline format/Undefined TypeError: at Function.Timeline.GregorianDateLabeller.getMonthName

On certain Chrome browsers (all have the same, v70.0.3538.77 (Official Build) (64-bit), version ) we have this bug (already abandoned report of 2013: https://github.com/simile-widgets/ancient-simile-widgets/issues/494): mw.loader.implement.css @ VM66:200 load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=vector&version=0l60dw6:52 jQuery.Deferred exception: Cannot read property '1' of undefined TypeError: Cannot read property '1' of undefined at Function.Timeline.GregorianDateLabeller.getMonthName (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:79:997) at Timeline.GregorianDateLabeller.defaultLabelInterval (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:81:426) at Timeline.GregorianDateLabeller.labelInterval (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:80:253) at Timeline.EtherIntervalMarkerLayout.createIntervalMarker (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:77:340) at Timeline.GregorianEtherPainter.paint (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:64:879) at Timeline._Band.paint (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:14:185) at Timeline._Band.layout (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:14:114) at Timeline._Band._recenterDiv (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:22:690) at Timeline._Band.setViewLength (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:15:107) at Timeline._Impl._distributeWidths (http://server.com/wiki/load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:8:965) undefined jQuery.Deferred.exceptionHook @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=vector&version=0l60dw6:52 load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=vector&version=0l60dw6:52 Uncaught TypeError: Cannot read property '1' of undefined at Function.Timeline.GregorianDateLabeller.getMonthName (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:79) at Timeline.GregorianDateLabeller.defaultLabelInterval (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:81) at Timeline.GregorianDateLabeller.labelInterval (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:80) at Timeline.EtherIntervalMarkerLayout.createIntervalMarker (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:77) at Timeline.GregorianEtherPainter.paint (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:64) at Timeline._Band.paint (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:14) at Timeline._Band.layout (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:14) at Timeline._Band._recenterDiv (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:22) at Timeline._Band.setViewLength (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:15) at Timeline._Impl._distributeWidths (load.php?debug=false&lang=en&modules=ext.smile.timeline.core&skin=vector&version=1tg42br:8)

Any ideas what is causing it to fail?