CODEWITHSUNDEEP
c++c++11c++-chrono
David
David

Reputation: 28178

Can calling system_clock::now() in the future give a time in the past?

Can calling system_clock::now() in the future give a time in the past? For example, because of daylight savings time?

What about calling system_clock::now().time_since_epoch()? Or is it guaranteed to always return later and later time points? Even without a guarantee, do all implementations do it a certain way in practice (if so, how)?

Upvotes: 3

Views: 248

Answers (1)

Olipro
Olipro

Reputation: 3529

std::chrono offers both system_clock and steady_clock for general-purpose time tracking.

The system_clock class will rely entirely on the platform and thus if a user were to change the system time (or NTP etc.) it will obviously send you into the past/future. User changing timezone will not affect you providing you're not using local time.

steady_clock on the other hand is completely consistent (although VS2013 misimplemented this and it behaves just like system_clock, sadly)

Thus, if you need to know absolute time rather than simply the passing of time, I would suggest that you grab a value from system_clock at startup and use steady_clock from that point onwards.

Upvotes: 2

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