I've just had the following response from Hikvision support:
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Dear customer
Thanks for the provided details, we would suggest upgrading the device's firmware to latest one which can be found in the following:
http://www.hikvisioneurope.com/uk/portal/?dir=portal/Product Firmware/Hikvision/04 Back Ends/NVR/[76 77 96 NI-I]/V4.51.007 build210818
A default on the system is required in order to get the new firmware running correctly, if that is not done you still may get the same issue after the upgrade.
PS.
As the end user of our products - we kindly suggest contacting you supplier which is your fisrt line support for any technical issue. Note that we only support CCTV installation companies and product distributors, in turn - end users will get help from these chains.
Kindest Regards
Andre
Technical Support Engineer
UK & Ireland
4 The Square, Stockley Park, Uxbridge, UB11 1ET
Tel: + 44(0)1628 902140
Email:
support.uk@hikvision.com
Website:
www.hikvision.com/uk
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Clearly, they don't want the people who ultimately purchase and use their products to get direct support!
The link points to their latest firmware version, which was added only two days ago, so I hadn't used this when I updated my firmware three days ago!
I have now applied this most recent 210818 build and it appears to have fixed the issue. When I now set DST Bias to 60 mins, the Device Time stays at the correct time, rather than being updated to 30 mins into the future.
I wonder why Hikvision hadn't previously made this firmware version available, unless it was just coincidence it appeared when I raised this issue.
EDIT.
PS. The iVMS-4200 software didn't detect a Firmware Upgrade for my box and states "No available version" (even before I upgraded). Also, the "Online Upgrade" page of the Maintenance menu for the NVR didn't detect any further firmware versions when I clicked "Check Upgrade":
It would appear this latest firmware version hasn't been properly registered to allow the software to detect it or the Hikvision software is just very flaky!