Make absl::Now() monotonic

[sfc-gh-mvashishtha]

Could we make absl::Now() monotonic while preserving its performance? It would be really convenient to get a wall clock time that's monotonic. How does absl::Now() compare in performance to the approach of

1. saving the system time once (e.g. with std::chrono::system_clock() ) along with a std::chrono::steady_clock() time.
2. calculating the delta in std::chrono::steady_clock() and adding it to the saved system clock time.

Related issue: #172

[derekmauro]

The primary purpose of absl::Now() (and std::chrono::system_clock::now()) is to return the absolute, real-world time (wall-clock). This time is intentionally not monotonic because it's susceptible to:

• NTP Updates: Network Time Protocol adjustments that can cause the clock to jump forward or backward.
• Manual Changes: A system administrator changing the date/time.

If we modified absl::Now() to be monotonic, we would break its core contract - it would no longer reliably represent the system's actual wall-clock time, which would cause confusion for every downstream consumer (logging, expiry checks, file timestamps, etc.).

We should not make absl::Now() monotonic. We should use the appropriate clock for the specific need.

By the way, this question seems like an example of X-Y problem-solving, where the question is about a proposed solution (Y: making absl::Now() monotonic) instead of clearly stating the underlying problem (X: Why do you need a monotonic wall-clock time?)

[sfc-gh-mvashishtha]

@derekmauro the context is that I want to measure the time between two events as an absl::Duration, but with a monotonic clock, so that I can be sure that I get the actual duration between the events. It's convenient that operator- is defined for absl::Time to return absl::Duration, but there's no operator- for times from a steady clock that returns an absl::Duration. I am fixing a bug where a few places in a codebase are subtracting two times from absl::Now() to get a duration, and sometimes recording a negative duration. I understand that it's incorrect to expect absl::Now() to be monotonic, and that we can wrap absl::Now() in a monotonic timer, but abseil users might still expect the duration between two absl::Now() calls to be accurate.

[derekmauro]

This is how I would do this:

auto start = std::chrono::steady_clock::now();
auto end = std::chrono::steady_clock::now();
absl::Duration elapsed = absl::FromChrono(end - start);