Hi Cornelius,
a) A, B, C, D, F nodes use formula EF = ES + Duration - 1. E node uses formula EF = ES + Duration. Both formulas are ok, but please use it consistently for all nodes.
b) The slack time, also known as float time, for an activity is the time between the earliest and latest start time. Critical activities, activities on the critical path, will always have zero slack.
Slack = LF - EF
Slack = LS - ES
The only node on your diagram that fulfill both formulas is D. The rest look incorrect. Let me refer to Figure 6-16 in PMBoK guide (6-th ed).
Regards,
Ruslan.