Both the date/time and the newgrounds plugins are great, but have a few prerequisites. I will write about it as well, but my goal is to keep things as basic as possible, hopefully working "out of the box" in terms of plugins. I want to go over each available method and the advantages/disadvantages of each. Here's what I have so far.
ExecJS("Date()")
- Simplest solution without plugin
- Works on system time, gameable
- Need to convert to UNIX Time, complicated
EDIT - Found out ExecJS("Date.now()") gives the timestamp without needing to convert.
Rex_Date plugin
- Simplest solution with plugin
- System time
- Needs plugin
Geocache timestamp
- System time
- Requires location services - intrusive to user
Multiplayer Plugin
- Need to set up own server to request server's timestamp
- Advanced/complicated
AJAX request to third party website
- Simplest online/server solution
- Relies on availability of third party website
- May cause unwelcome stress to server
- Possibly requires authentication/plugin
NG.io
- Requires plugin, NG.io account/project