adding 2 short duration events causes a JavaScript error

Steps to Reproduce

1. open chrome dev tool
2. load the attached zul

Current Result

a javascript shows in Console tab:

calendar.wpd:2346 Uncaught TypeError: Cannot read property 'indexOf' of undefined
at init.fixPosition (calendar.wpd:2346)
at init._rePositionDay (calendar.wpd:1601)
at init._zkf (calendar.wpd:2374)
at _invoke (zk.wpd:12532)
at init.fire (zk.wpd:12515)
at _fire (zk.wpd:12658)
at Object.fireDown (zk.wpd:12753)
at Object.fireDown (zk.wpd:14059)
at Object.fireSized (zk.wpd:14570)
at init.replaceHTML (zk.wpd:16256)

Expected Result

no error.

Root cause

getTimeIndex() returns an index out of number of total slots index (48 > 47)

workaround

include zkcal-70.js on a page with Calendar