The patch comes with a detailed document explaining what it will introduce and what needs to be done when it is installed.
For DST you will have to apply 1)OS patch to laptops / desktops 2)OS patch to servers 3)OS patch to java as SP 19 required java 1.4.x 4)Patch for OVSD application servers
Can I just set system time manually and not worry about installing the patch?
Taking the above into consideration it will just affect the OS and not not the application, clients and java. Also wont the server be taking the time from the domain so it could revert to the domain time.
You can also ask HP support if they can create a DST patch for SP19. We were able to obtain one for SP20, since we were worried about the speed for SP21 and did not want to move up.
If you do not apply the patch, my guess would be that the times on Service Desk would be off by a hour but I'm not sure if the client would see that or not. Service Desk times are stored in the database as GMT so on March 11, if your timezone changes from being 6 hours behind GMT to being 5 hours behind, this would impact the value the time is stored in the database relative to GMT.
Will the client see anything different? I'm not sure since both the client and the server may be off by the same amount, it's possible that they could be incorrect but working together although the idea behind the update is that everything involved needs to agree on the time and the offset from GMT. If the server thinks its 5 hours behind GMT on March 11 but the client still believes its 6 hours behind, I would suspect that there would be problems with incorrect dates and times, especially in notifications. I think everyone agrees that there is some consequence to not upgrading but it's difficult to know exactly what the consequences are.
I think your best bet is to talk to HP about a patch for SP19. That would be the easiest and safest solution.