CS253 Chrono

Roman Timekeeping

Time in Real Life

Our timekeeping notation is one of the worst systems ever devised.
If you proposed such a system, you’d get laughed out of the room.
Consider numbers: we use base 10 notation. Each digit is worth ten times more than the digit to its right. Base 10 is a bit weird, but we have that many fingers. ✋️🤚 It works ok.
Imagine what a disaster it would be if each digit place used a different multiplier than 10. Some are zero-based some are one-based, some are words, and some places have multiple definitions.
Real-life timekeeping is like that.

Time in Real Life

The common timekeeping system in the United States:

Base unit: seconds (0…59) except for leap seconds
Next unit: minutes (0…59)
Next unit: hours (0…23 or 12AM, 1AM…11AM, 12PM, 1PM…11PM) except for leap days
Next unit: days (Mon…Sun or Sun…Sat, no generally accepted numbers)
Next unit: weeks (0…53 or 1…54)
-or-
Next unit: months (Jan…Dec or 1…12)
Next unit: years (…, 3BC, 2BC, 1BC, 1AD, 2AD, 3AD, …, 2024AD, …) or BCE/CE

Time in C++

Given that monstrosity, time software is going to be complicated, because timekeeping in real life is complicated. There’s no way around that.

Time in C++

overhead: #include <chrono>
- This introduces a lot of new symbols, quarantined to their own namespace std::chrono to avoid conflicts, so put using namespace std::chrono; in your code.
duration: a length of time
- 75 minutes
- 3.456μs
time_point: an instant in time
- November 21, 2024, at 11:19 PM
- the launch of Sputnik
- 2021‒01‒20T12:00:00-0500
clock: an epoch + units
- CSU week
- Julian day
- Linux time_t

duration

A duration is not much more than a number and units: 3ms, 1.25 hours, 100 years
These are stored in the tempated class duration:
- template<typename Rep, class Period>
- Rep is a representation: int, long, double, etc.
- Period is a number of seconds, expressed as a Ratio type:
  - Ratio<1,1> (seconds)
  - Ratio<1,1000> (milliseconds)
  - Ratio<60*60,1> or Ratio<60*60> (hours)
duration::count() returns the count.
The Period is a compile-time construct. This enables calculations to convert between different periods (hours, ms) to be mostly done at compile-time, not at run-time.

Predefined duration types

The following types are predefined. They might not actually be long, but they’re an integer type that is big enough.

    using hours	       = duration<long, ratio<60*60,1>>;
    using minutes      = duration<long, ratio<60,1>>;
    using seconds      = duration<long, ratio<1,1>>;
    using milliseconds = duration<long, ratio<1,1000>>;
    using microseconds = duration<long, ratio<1,1'000'000>>;
    using nanoseconds  = duration<long, ratio<1,1'000'000'000>>;

C++20 adds days, weeks, months, and years. Perhaps their variable nature (DST, leap years) caused a delay in their standardization.

duration constants

Several suffixes exist for duration constants:
- number h (hours)
- number min (minutes) (as if C++ doesn’t have enough min already)
- number s (seconds) (as opposed to "string object "s)
- number ms (milliseconds)
- number us (microseconds) (alas, us, not μs)
- number ns (nanoseconds)

12h ≡ hours(12), 123ms ≡ milliseconds(123), etc.

if (0.5h - 15min == 900s && seconds(5) == 5000000us)
    cout << "Hooray!\n";

Hooray!

Extracting the units

Even though 1h == 60min, they are represented differently:

auto a = 1h;
auto b = 60min;
if (a == b)
    cout << a.count() << ' ' << b.count();

1 60

This works because duration comparison via == is smart. It doesn’t just naïvely compare the .count() values—it scales them according to their ratios. Probably some sort of Least Common Multiple business.

We’re comfortable with 3 == 3.0, even though they’re represented differently. This is just more of the same.

Fun with durations

// How long is a semester, in seconds?
using weeks = duration<long, ratio<7*24*60*60>>; // until C++20
weeks w(15);
seconds s = w;
cout << w.count() << " weeks is " << s.count() << " seconds.\n";

15 weeks is 9072000 seconds.

w is, essentially, a long containing 15.
- It also has compile-time information of the ratio of weeks to seconds (604800:1).
s is, essentially, a long containing 9072000.
- It also has compile-time information of the ratio of seconds to seconds (1:1).

Define your own units

“In the future, everyone will be world-famous for 15 minutes.”
—Andy Warhol

using warhol = duration<float, ratio<15*60>>;
warhol teaching(75min);
cout << "Famous for " << teaching.count() << " Warhols.\n";

Famous for 5 Warhols.

Duration conversion

Durations can be assigned, sometimes:

milliseconds ms = 2s;
cout << ms.count();

but not if there would be a loss of precision:

seconds s = 1999ms;  // 1s or 2s?
cout << s.count();

c.cc:1: error: conversion from 'duration<[...],ratio<[...],1000>>' to 
   non-scalar type 'duration<[...],ratio<[...],1>>' requested

duration_cast()

To force a loss-of-precision assignment, use duration_cast(), which truncates (discards the fractional part):

seconds s = duration_cast<seconds>(4567ms);
cout << s.count();

or use floating-point storage:

using floatsecs = duration<double, ratio<1,1>>;
floatsecs s = 5678ms;
cout << s.count();

5.678

If the storage type of a duration is a floating-point type, the type system figures that you’ve got it covered.

Dates are tricky!

How many days in a year? 365, of course! Except …

Add a leap day (February 29) in years divisible by 4.
However, years divisible by 100 are not leap years
However, years divisible by 400 are leap years.

So, an average year is 365 + 1⁄4 − 1⁄100 + 1⁄400 = 365.2425 days, or 365.2425 × 24 × 60 × 60 = 31556952 seconds.

Even this value is not exact. Why? Because the time it takes the Earth to go around the Sun is not a even multiple of the time it takes the Earth to turn once. Nice solar system, divine creator(s)!

Good enough?

Let’s see how big a quantity of nanoseconds we can represent, and translate that to years:

constexpr long spy = 365.2425 * 24 * 60 * 60; // seconds/year
auto nsmax = nanoseconds::max();
cout << "Max ns: " << nsmax.count() << '\n';
using dyears = duration<double, ratio<spy>>;
dyears y = nsmax;
cout << "Max ns in years: " << y.count() << '\n';

Max ns: 9223372036854775807
Max ns in years: 292.277

Good enough.

time_point

A time_point is a point in time, an epoch:
- creation of the world
- birth of some popular dude
- the founding of the city or nation
- beginning of the semester
- January 1, 1970
and a duration, which we’ve discussed above.

time_point examples:

Midterm #2 (start of CSU semester + 10 weeks)
American Revolution (Common Era start + 1776 years)

Clocks

C++ defines several clocks, which yield time_point values.
system_clock
- Think “wall clock”.
- Measure actual time, like 11:19ᴘᴍ, and historic things like dates & ages.
- Might have crude precision, e.g., seconds.
- Epoch is assumed to be a significant time in the past, e.g., 1970.
steady_clock
- Think “real-time clock”.
- Measure elapsed time—how long a program runs. May only be able to deal with a small total amount of time.
- Assumed to have good precision, e.g., nanoseconds.
- Epoch may be recent, e.g., system boot.

Using a clock

A clock has a ::now() static method that yields a time_point representing now.
A time_point has a .time_since_epoch() method that yields a duration since the epoch, in whatever units the clock naturally deals with.

constexpr long spy = 365.2425 * 24 * 60 * 60;  // seconds/year
using dyears = duration<double, ratio<spy>>;
auto now = system_clock::now();     // a time_point
dyears y = now.time_since_epoch();  // a duration
cout << y.count() << " years since the epoch.\n";

54.893 years since the epoch.

Using a clock

system_clock has a static method ::to_time_t(), which converts a time_point to seconds since the Linux time_t epoch (generally the start of 1970).

auto now = system_clock::now();                 // time_point
time_t t = system_clock::to_time_t(now);        // time_t
cout << t << " seconds since 1970 started.\n";
cout << ctime(&t);                              // char *

1732256361 seconds since 1970 started.
Thu Nov 21 23:19:21 2024

Age

How old is your instructor? Even that can be represented!

constexpr long spy = 365.2425 * 24 * 60 * 60;  // seconds/year
using dyears = duration<double, ratio<spy>>;
using months = duration<long, ratio<spy,12>>;  // till C++20
using days = duration<long, ratio<24*60*60>>;  // till C++20

auto birth = system_clock::from_time_t(-0x16e06d50); // 🍼👶
auto age = system_clock::now() - birth;

cout << duration_cast<days>(age).count()   << " days\n"
     << duration_cast<months>(age).count() << " months\n"
     << dyears(age).count() << " years\n";

24491 days
804 months
67.0554 years

2038‒01‒18T20:14:08

In January 2038, Linux time (seconds since the start of 1970) will reach 2³¹, or 8000 0000₁₆. This will cause problems with systems that store time_t as a 32-bit signed integer.

typedef duration<double,ratio<60*60*24>> ddays;   // until C++20
auto tp = system_clock::from_time_t(0x7fffffff);  // 2038‒01‒18
ddays ndays = tp - system_clock::now();
cout << ndays.count() << " days until the y2038 bug!\n"
     << "My time_t is " << numeric_limits<time_t>::digits << " bits.\n";

4805.87 days until the y2038 bug!
My time_t is 63 bits.

Timing

Use steady_clock to time code:

auto start = steady_clock::now();                // time_point
for (long i=0; i<1e8; i++);                      // count to 10⁸
auto elapsed = steady_clock::now() - start;
auto ms = duration_cast<milliseconds>(elapsed);  // duration
cout << "That took " << ms.count() << "ms.";

That took 193ms.

steady_clock most likely has much better than millisecond resolution, so we duration_cast() to milliseconds.

Reminder

Don’t forget using namespace std::chrono; in your code!

CS253: Software Development with C++

Spring 2021

Chrono

CS253 Chrono

Time in Real Life

Time in Real Life

Time in C++

Time in C++

duration

Predefined duration types

duration constants

Extracting the units

Fun with durations

Define your own units

Duration conversion

duration_cast()

Dates are tricky!

Good enough?

time_point

Clocks

Using a clock

Using a clock

Age

2038‒01‒18T20:14:08

Timing

Reminder