Non-working intervals are calculated incorrectly in certain time zone

[bmblb]

Reported here: https://forum.bryntum.com/viewtopic.php?f=52&t=30056&p=152506#p152506

Steps to reproduce:

1. use this dataset with gantt advanced demo: launch-saas.json
2. in app.js comment out view start/end dates
3. set system time zone to Cairo time

Run the demo and see a number of console errors