Slovakia zone is the same as Czech zone and start=3/-1/7/2;end=10/-1/7/2;save=1 works for me. So problem seems to be elsewhere. There are several setting that affects time offset, including the options delivered via DHCP.
You need to decide either to honor DHCP or maintain local corrections but not both.
Recommended settings for you (I recommend not to honor DHCP as it announce time-zone offset incorrectly in many cases):
Ignore DHCP Time Offset=Yes
Daylight Saving Time Rule=Yes
Time Offset (HH/mm)="" (empty)
Time Zone=GMT+0100
Daylight Saving Time Rule="start=3/-1/7/2;end=10/-1/7/2 ;save=1"
Hope it help.