Clocks |

A clock is an object which knows about a particular point on the time axis known as now. All times before that point are considered in the past and times beyond it are in the future.

## Abstract Clocks

`DT_CLOCK` is a deferred class offering features such as `time_now`, `date_now` and `date_time_now` which can provide today's date and the current time according to the clock's criterion. Three other features, `set_time_to_now`, `set_date_to_now` and `set_date_time_to_now`, allow times, dates and date/times to be reset to the clock's current time.

## System Clocks

Typically programs need to access the current time of the computer they are running on. That's what class `DT_SYSTEM_CLOCK``` is for. This class is an heir of class `DT_CLOCK` and implements its deferred features so that the current local time of the system is returned. The typical way to invoke the system clock is to inherit from `DT_SHARED_SYSTEM_CLOCK``` and call the once function `system_clock`. Alternatively, class `DT_UTC_SYSTEM_CLOCK``` provides the current UTC (Coordinated Universal Time) time of the system. It is available through the once function `utc_system_clock` in class `DT_SHARED_SYSTEM_CLOCK`.

`DT_SYSTEM_CLOCK` and `DT_UTC_SYSTEM_CLOCK` are implemented using the time facilities provided by each supported Eiffel compiler. The corresponding clusters have been added to `$``GOBO/library/kernel.xace`. There is therefore no other addition needed to the Xace files.

There is an example using system clock in `$GOBO/library/time/example/clock`.

## Customizable Clocks

With the design of the clock classes, it is quite easy to write user-customized clocks by inheriting from `DT_CLOCK` and giving an implementation to the deferred features `set_time_to_now`, `set_date_to_now` and `set_date_time_to_now`. An example of a possible customizable clock is a clock which could run twice faster than the regular system clock for instance. This kind of clock can be useful in order to run simulation tests driven by time events. If the clock runs much faster than the system clock then tests which may normally last several hours or days will finish in a reasonable amount of time. Another application of customizable clocks is a clock which runs at the same speed as the system clock, but returns dates and times in the past or in the future. This could have been used for example to check that a program was Y2K compliant just by readjusting the clock to just before midnight on 31 December 1999. There are certainly many other examples where customizable clocks can be useful.