The most important action first - put the connection session in UTC mode:
set time_zone = '+00:00';
After this is done, both NOW() and UTC_TIMESTAMP() return the same value. The FROM_UNIXTIME() call returns UTC instead of local datetime and UNIX_TIMESTAMP() returns the seconds since 1970-01-01 UTC (called Unix epoch).
To set the default timezone:
To get a DateTime object from a Unix timestamp:
$date = new DateTime('@' . $seconds);
In the above constructor, the Unix timestamp is always interpreted as UTC time, even if you specify a timezone as the second argument.
To get back some nice format:
echo $date->format('Y-m-d H:i:s');
SQLite is by default in UTC. So both datetime('now') and datetime([seconds], 'unixepoch') return a UTC datetime.
To get the milliseconds since Unix epoch:
var date = new Date(); date.getTime();
Both methods return milliseconds and not seconds!
When you use the constructor that accepts milliseconds, it is always interpreted as UTC time:
var date = new Date(ts);
When you call the constructor that accepts year, month, etc. it is always interpreted as local time:
var date = new Date(2014, 9, 8, 14, 30, 0);
If you want to convert some UTC datetime to a Date object, you can use the Date.UTC() method:
var date = new Date(Date.UTC(2014, 9, 8, 14, 30, 0));