D.8 Monotonic Time

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This clause specifies a high-resolution, monotonic clock package.

Static Semantics

The following language-defined library package exists:

package Ada.Real_Time is

    type Time is private;
    Time_First : constant Time;
    Time_Last : constant Time;
    Time_Unit : constant := implementation-defined-real-number;

    type Time_Span is private;
    Time_Span_First : constant Time_Span;
    Time_Span_Last : constant Time_Span;
    Time_Span_Zero : constant Time_Span;
    Time_Span_Unit : constant Time_Span;

    Tick : constant Time_Span;
    function Clock return Time;

    function "+" (Left Time; Right Time_Span) return Time; 
    function "+" (Left Time_Span; Right Time) return Time; 
    function "-" (Left Time; Right Time_Span) return Time; 
    function "-" (Left Time; Right Time) return Time_Span;

    function "<" (Left, Right Time) return Boolean; 
    function "<="(Left, Right Time) return Boolean; 
    function ">" (Left, Right Time) return Boolean; 
    function ">="(Left, Right Time) return Boolean;

    function "+" (Left, Right Time_Span) return Time_Span; 
    function "-" (Left, Right Time_Span) return Time_Span; 
    function "-" (Right Time_Span) return Time_Span; 
    function "*" (Left Time_Span; Right Integer) return Time_Span; 
    function "*" (Left Integer; Right Time_Span) return Time_Span; 
    function "/" (Left, Right Time_Span) return Integer; 
    function "/" (Left Time_Span; Right Integer) return Time_Span;

    function "abs"(Right Time_Span) return Time_Span;

This paragraph was deleted.

    function "<" (Left, Right Time_Span) return Boolean; 
    function "<="(Left, Right Time_Span) return Boolean; 
    function ">" (Left, Right Time_Span) return Boolean; 
    function ">="(Left, Right Time_Span) return Boolean;

    function To_Duration (TS Time_Span) return Duration; 
    function To_Time_Span (D Duration) return Time_Span;

    function Nanoseconds  (NS Integer) return Time_Span;     
    function Microseconds (US Integer) return Time_Span; 
    function Milliseconds (MS Integer) return Time_Span;

    type Seconds_Count is range implementation-defined;

    procedure Split(T in Time; SC out Seconds_Count; TS out Time_Span); 
    function Time_Of(SC Seconds_Count; TS Time_Span) return Time;

    ... -- not specified by the language
end Ada.Real_Time;

In this Annex, real time is defined to be the physical time as observed in the external environment. The type Time is a time type as defined by 9.6; values of this type may be used in a delay_until_statement. Values of this type represent segments of an ideal time line. The set of values of the type Time corresponds one-to-one with an implementation-defined range of mathematical integers.

The Time value I represents the half-open real time interval that starts with E+I*Time_Unit and is limited by E+(I+1)*Time_Unit, where Time_Unit is an implementation-defined real number and E is an unspecified origin point, the epoch, that is the same for all values of the type Time. It is not specified by the language whether the time values are synchronized with any standard time reference. For example, E can correspond to the time of system initialization or it can correspond to the epoch of some time standard.

Values of the type Time_Span represent length of real time duration. The set of values of this type corresponds one-to-one with an implementation-defined range of mathematical integers. The Time_Span value corresponding to the integer I represents the real-time duration I*Time_Unit.

Time_First and Time_Last are the smallest and largest values of the Time type, respectively. Similarly, Time_Span_First and Time_Span_Last are the smallest and largest values of the Time_Span type, respectively.

A value of type Seconds_Count represents an elapsed time, measured in seconds, since the epoch.

Dynamic Semantics

Time_Unit is the smallest amount of real time representable by the Time type; it is expressed in seconds. Time_Span_Unit is the difference between two successive values of the Time type. It is also the smallest positive value of type Time_Span. Time_Unit and Time_Span_Unit represent the same real time duration. A clock tick is a real time interval during which the clock value (as observed by calling the Clock function) remains constant. Tick is the average length of such intervals.

The function To_Duration converts the value TS to a value of type Duration. Similarly, the function To_Time_Span converts the value D to a value of type Time_Span. For both operations, the result is rounded to the nearest exactly representable value (away from zero if exactly halfway between two exactly representable values).

To_Duration(Time_Span_Zero) returns 0.0, and To_Time_Span(0.0) returns Time_Span_Zero.

The functions Nanoseconds, Microseconds, and Milliseconds convert the input parameter to a value of the type Time_Span. NS, US, and MS are interpreted as a number of nanoseconds, microseconds, and milliseconds respectively. The result is rounded to the nearest exactly representable value (away from zero if exactly halfway between two exactly representable values).

The effects of the operators on Time and Time_Span are as for the operators defined for integer types.

The function Clock returns the amount of time since the epoch.

The effects of the Split and Time_Of operations are defined as follows, treating values of type Time, Time_Span, and Seconds_Count as mathematical integers. The effect of Split(T,SC,TS) is to set SC and TS to values such that T*Time_Unit = SC*1.0 + TS*Time_Unit, and 0.0 <= TS*Time_Unit < 1.0. The value returned by Time_Of(SC,TS) is the value T such that T*Time_Unit = SC*1.0 + TS*Time_Unit.

Implementation Requirements

The range of Time values shall be sufficient to uniquely represent the range of real times from program start-up to 50 years later. Tick shall be no greater than 1 millisecond. Time_Unit shall be less than or equal to 20 microseconds.

Time_Span_First shall be no greater than -3600 seconds, and Time_Span_Last shall be no less than 3600 seconds.

A clock jump is the difference between two successive distinct values of the clock (as observed by calling the Clock function). There shall be no backward clock jumps.

Documentation Requirements

The implementation shall document the values of Time_First, Time_Last, Time_Span_First, Time_Span_Last, Time_Span_Unit, and Tick.

The implementation shall document the properties of the underlying time base used for the clock and for type Time, such as the range of values supported and any relevant aspects of the underlying hardware or operating system facilities used.

The implementation shall document whether or not there is any synchronization with external time references, and if such synchronization exists, the sources of synchronization information, the frequency of synchronization, and the synchronization method applied.

The implementation shall document any aspects of the external environment that could interfere with the clock behavior as defined in this clause.


For the purpose of the metrics defined in this clause, real time is defined to be the International Atomic Time (TAI).

The implementation shall document the following metrics:

  • An upper bound on the real-time duration of a clock tick. This is a value D such that if t1 and t2 are any real times such that t1 < t2 and Clockt1 = Clockt2 then t2 - t1 <= D.
  • An upper bound on the size of a clock jump.
  • An upper bound on the drift rate of Clock with respect to real time. This is a real number D such that

E*(1-D) <= (Clockt+E - Clockt) <= E*(1+D)
provided that: Clockt + E*(1+D) <= Time_Last.
  • where Clockt is the value of Clock at time t, and E is a real time duration not less than 24 hours. The value of E used for this metric shall be reported.
  • An upper bound on the execution time of a call to the Clock function, in processor clock cycles.
  • Upper bounds on the execution times of the operators of the types Time and Time_Span, in processor clock cycles.

Implementation Permissions

Implementations targeted to machines with word size smaller than 32 bits need not support the full range and granularity of the Time and Time_Span types.

Implementation Advice

When appropriate, implementations should provide configuration mechanisms to change the value of Tick.

It is recommended that Calendar.Clock and Real_Time.Clock be implemented as transformations of the same time base.

It is recommended that the best time base which exists in the underlying system be available to the application through Clock. Best may mean highest accuracy or largest range.


30  The rules in this clause do not imply that the implementation can protect the user from operator or installation errors which could result in the clock being set incorrectly.

31  Time_Unit is the granularity of the Time type. In contrast, Tick represents the granularity of Real_Time.Clock. There is no requirement that these be the same.

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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual