Wronge start / finish date

I set the project calender to 12 Hrs/ Day 7 Day/weeks and also the admin user pereference to 12 Hrs / day and check all user to specify the number of hous for each time period .

my problem is when I try to make - 6 hours lag for an activity it give me wronge start date for it

for example the activty by default start at 21 spep. 2PM and after assign -6 Hrs alg it start at 19 sep 10 AM ?