I've seen plenty of questions about how to get the date/time in JavaScript, and the answers always look something like

Date.now() returns the UTC timestamp in milliseconds

But how reliable is that value really? Is it just based on the system clock of whatever puter is running the browser? If so, it seems like it could vary arbitrarily far from true UTC time, but how much variation should be expected?. I know caring about accurate milliseconds is a lost cause, but what about on the order of seconds or minutes?

I've seen plenty of questions about how to get the date/time in JavaScript, and the answers always look something like

Date.now() returns the UTC timestamp in milliseconds

But how reliable is that value really? Is it just based on the system clock of whatever puter is running the browser? If so, it seems like it could vary arbitrarily far from true UTC time, but how much variation should be expected?. I know caring about accurate milliseconds is a lost cause, but what about on the order of seconds or minutes?

• javascript
• date
• timestamp
• utc

• 1 Unless you're calling out to an external clock, all bets are off. Even then, it's sketchy. – Will Jenkins Commented Jul 19, 2019 at 22:24

3 Answers 3

Is it just based on the system clock of whatever puter is running the browser?

Right. It may well be pletely inaccurate. For example, on my puter, whose clock is inaccurate, at this moment, Date.now() returns 1563724931361, which, when passed into new Date, gives:

Sun Jul 21 2019 11:02:11 GMT-0500 (Central Daylight Time)

Which is pletely wrong.

If the client wishes to present an inaccurate Date.now(), it's pretty trivial for them to do so, though usually, for most ordinary users, it'll be accurate, because most people have accurate clocks.

How accurate? It depends on the hardware, on how long it was since their puter last requested the time from a time server, and on how long since the puter was turned on (either from power off or standby/hibernate), but most times, it won't be off by more than a minute.

There is always ECMA-262:

20.3.3.1 Date.now ( )

The now function returns a Number value that is the time value designating the UTC date and time of the occurrence of the call to now.

That's it, there's no requirement for where the value es from or its accuracy, so it's implementation dependent. In practice it seems that most implementations use values from the host system, so reliability is no better than what might be expected of a system clock and user provided settings such as calendar, timezone, DST, current time, etc.

In the light of side-channel memory attacks the accuracy of the timing sources in a browser were limited. According to current information, from W3C High Resolution Time issue tracker, the limitation is a little different between different browsers:

• Mobile Chrome + Edge: 100us + 100us jitter
• Desktop Chrome: 5us
• Safari + Firefox: 1ms