I'm running LR CC 2015 on a 27" retina iMac and when I try to batch edit timestamps, it only updates the timestamp of the first photo selected. Here the steps to reproduce:
1) Select 2 or more photos
2) Under metadata click the button to the right of Capture Time
3) In the Edit Capture Time dialog box, select the radio "Shift by set number of hours (time zone adjust)"
4) Click the drop down in the right side of the New Time box and select -19 hours as the time offset.
5) Click change
All the photos selected in step 1 should have their timestamps by -19 hours
Only the first photo of the set selected in step 1 has its timestamp adjusted.
That did it, thanks!