Clarification.... I wrote:
The problem is made complicated by Daylight Savings Time.
GMT is not adjusted for DST.
[....]
Alternatively, you could have a cell (e.g. D1) with 0
(not DST) or 1 (DST), then use the following formula:
=LEFT(A1,19)-TIME(8-D1,0,0)
[...or...]
=LEFT(A1,FIND(" ",A1,1+FIND(" ",A1)))-TIME(8-D1,0,0)

You should then copy the converted times and paste-special-value so that you
have constant times again.