ServerTimestamp vs Timestamp

What is the difference between serverTimestamp and Timestamp? Should I trust that Timestamp (in GMT) is the correct value to use for analyzing when users perform a specific action?

Example: I want pull all events for August 30th, 2016 from my website on Piwik which is set to Pacific Standard Time. Will all these events return according to August 30th GMT or PST? And can I trust timestamp to be the correct value?