HP OpenVMS Systems Documentation

This chapter describes the types of system time operations performed by the operating system and contains the following sections:

Section 27.1 describes the system time format.

Section 27.2 describes time conversion and date/time manipulation.

Section 27.3 describes how to get the current date and time and set the current time.

Section 27.4 describes how to set and cancel timer requests and how to schedule and cancel wakeups.

Section 27.5 describes using run-time library (RTL) routines to collect timer statistics.

Section 27.6 describes using date/time formatting routines.

Section 27.7 describes the Coordinated Universal Time (UTC) system.

27.1 System Time Format

The operating system maintains the current date and time in 64-bit format. The time value is a binary number in 100-nanosecond (ns) units offset from the system base date and time, which is 00:00 o'clock, November 17, 1858 (the Smithsonian base date and time for the astronomic calendar). Time values must be passed to or returned from system services as the address of a quadword containing the time in 64-bit format. A time value can be expressed as either of the following:

• An absolute time that is a specific date or time of day, or both. Absolute times are always positive values (or 0).
• A delta time that is an offset from the current time to a time or date in the future. Delta times are always expressed as negative values.

If you specify 0 as the address of a time value, the operating system supplies the current date and time.

27.1.1 Absolute Time Format

The operating system uses the following format for absolute time. The full date and time require a character string of 23 characters. The punctuation is required.

The operating system uses the following format for delta time. The full date and time require a character string of 16 characters. The punctuation is required.

A delta time is maintained as an integer value representing an amount of time in 100-ns units.

27.2 Time Conversion and Date/Time Manipulation

This section presents information about time conversion and date/time manipulation features, and the routines available to implement them.

27.2.1 Time Conversion Routines

Since the timer system services require you to specify the time in a 64-bit format, you can use time conversion run-time library and system service routines to work with time in a different format. Run-time library and system services do the following:

• Obtain the current date and time in an ASCII string or in system format
• Convert an ASCII string into the system time format
• Convert a system time value into an ASCII string
• Convert the time from system format to integer values

Table 27-1 shows time conversion run-time and system service routines.

You can use the SYS$GETTIM system service to get the current time in internal format, or you can use SYS$BINTIM to convert a formatted time to an internal time, as shown in Section 27.3.2. You can also use the LIB$DATE_TIME routine to obtain the time, LIB$CVT_FROM_INTERNAL_TIME to convert an internal time to an external time, and LIB$CVT_TO_INTERNAL to convert from an external time to an internal time.

27.2.1.1 Calculating and Displaying Time with SYS$GETTIM and LIB$SUBX

Example 27-1 calculates differences between the current time and a time input in absolute format, and then displays the result as delta time. If the input time is later than the current time, the difference is a negative value (delta time) and can be displayed directly. If the input time is an earlier time, the difference is a positive value (absolute time) and must be converted to delta time before being displayed. To change an absolute time to a delta time, negate the time array by subtracting it from 0 (specified as an integer array) using the LIB$SUBX routine, which performs subtraction on signed two's complement integers of arbitrary length. For the absolute or delta time format, see Section 27.1.1 and Section 27.1.2.

   .
   .
   .
! Internal times
! Input time in absolute format, dd-mmm-yyyy hh:mm:ss.ss
!
INTEGER*4 CURRENT_TIME (2),
2         PAST_TIME (2),
2         TIME_DIFFERENCE (2),
2         ZERO (2)
DATA ZERO /0,0/
! Formatted times
CHARACTER*23 PAST_TIME_F
CHARACTER*16 TIME_DIFFERENCE_F
! Status
INTEGER*4 STATUS
! Integer functions
INTEGER*4 SYS$GETTIM,
2         LIB$GET_INPUT,
2         SYS$BINTIM,
2         LIB$SUBX,
2         SYS$ASCTIM
! Get current time
STATUS = SYS$GETTIM (CURRENT_TIME)
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
! Get past time and convert to internal format
STATUS = LIB$GET_INPUT (PAST_TIME_F,
2                       'Past time (in absolute format): ')
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
STATUS = SYS$BINTIM (PAST_TIME_F,
2                    PAST_TIME)
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
! Subtract past time from current time
STATUS = LIB$SUBX (CURRENT_TIME,
2                  PAST_TIME,
2                  TIME_DIFFERENCE)
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
! If resultant time is in absolute format (positive value means
! most significant bit is not set), convert it to delta time
IF (.NOT. (BTEST (TIME_DIFFERENCE(2),31))) THEN
  STATUS = LIB$SUBX (ZERO,
2                    TIME_DIFFERENCE,
2                    TIME_DIFFERENCE)
END IF
! Format time difference and display
STATUS = SYS$ASCTIM (, TIME_DIFFERENCE_F,
2                    TIME_DIFFERENCE,)
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
TYPE *, 'Time difference = ', TIME_DIFFERENCE_F
END

If you are ignoring the time portion of date/time (that is, working just at the date level), the LIB$DAY routine might simplify your calculations. LIB$DAY returns to you the number of days from the base system date to a given date.

27.2.1.2 Obtaining Absolute Time with SYS$ASCTIM and SYS$BINTIM

The Convert Binary Time to ASCII String (SYS$ASCTIM) system service is the converse of the Convert ASCII String to Binary Time (SYS$BINTIM) system service. You provide the service with the time in the ASCII format shown in Section 27.3.2. The service then converts the string to a time value in 64-bit format. You can use this returned value as input to a timer scheduling service.

When you specify the ASCII string buffer, you can omit any of the fields, and the service uses the current date or time value for the field. Thus, if you want a timer request to be date independent, you could format the input buffer for the SYS$BINTIM service as shown in the following example. The two hyphens that are normally embedded in the date field must be included, and at least one blank must precede the time field.

#include <stdio.h>
#include <descrip.h>

/* Buffer to receive binary time */
struct {
        unsigned int buff1, buff2;
}binary_noon;

main()  {

        unsigned int status;
        $DESCRIPTOR(ascii_noon,"-- 12:00:00.00");  /* noon (absolute time) */

/* Convert time */
        status = SYS$BINTIM(&ascii_noon,        /* timbuf - ASCII time */
                 &binary_noon);                 /* timadr - binary time */

}

When the SYS$BINTIM service completes, a 64-bit time value representing "noon today" is returned in the quadword at BINARY_NOON.

27.2.1.3 Obtaining Delta Time with SYS$BINTIM

The SYS$BINTIM system service also converts ASCII strings to delta time values to be used as input to timer services. The buffer for delta time ASCII strings has the following format:

The first field, indicating the number of days, must be specified as 0 if you are specifying a delta time for the current day.

The following example shows how to use the SYS$BINTIM service to obtain a delta time in system format:

#include <stdio.h>
#include <descrip.h>

/* Buffer to receive binary time */
struct {
        unsigned int buff1, buff2;
}btenmin;

main()  {

        unsigned int status;
        $DESCRIPTOR(atenmin,"0 00:10:00.00");  /* 10-min delta */

/* Convert time from ASCII to binary */
        status = SYS$BINTIM(&atenmin,   /* timbuf - time in ASCII */
                &btenmin);              /* timadr - binary time */

}

If you are programming in VAX MACRO, you can also specify approximate delta time values when you assemble a program, using two MACRO .LONG directives to represent a time value in 100-ns units. The arithmetic is based on the following formula:

1 second = 10 million * 100 ns

For example, the following statement defines a delta time value of 5 seconds:

FIVESEC:  .LONG -10*1000*1000*5,-1 ; Five seconds

The value 10 million is expressed as 10*1000*1000 for readability. Note that the delta time value is negative.

If you use this notation, however, you are limited to the maximum number of 100-ns units that can be expressed in a longword. In time values this is slightly more than 7 minutes.

27.2.1.4 Obtaining Numeric and ASCII Time with SYS$NUMTIM

The Convert Binary Time to Numeric Time (SYS$NUMTIM) system service converts a time in the system format into binary integer values. The service returns each of the components of the time (year, month, day, hour, and so on) into a separate word of a 7-word buffer. The SYS$NUMTIM system service and the format of the information returned are described in the OpenVMS System Services Reference Manual.

You use the SYS$ASCTIM system service to format the time in ASCII for inclusion in an output string. The SYS$ASCTIM service accepts as an argument the address of a quadword that contains the time in system format and returns the date and time in ASCII format.

If you want to include the date and time in a character string that contains additional data, you can format the output string with the Formatted ASCII Output (SYS$FAO) system service. The SYS$FAO system service converts binary values to ASCII representations, and substitutes the results in character strings according to directives supplied in an input control string. Among these directives are !%T and !%D, which convert a quadword time value to an ASCII string and substitute the result in an output string. For examples of how to do this, see the discussion of $FAO in the OpenVMS System Services Reference Manual.

27.2.2 Date/Time Manipulation Routines

The run-time LIB$ facility provides several date/time manipulation routines. These routines let you add, subtract, and multiply dates and times. Use the LIB$ADDX and LIB$SUBX routines to add and subtract times, since the times are defined in integer arrays. Use LIB$ADD_TIMES and LIB$SUB_TIMES to add and subtract two quadword times. When manipulating delta times, remember that they are stored as negative numbers. For example, to add a delta time to an absolute time, you must subtract the delta time from the absolute time. Use LIB$MULT_DELTA_TIME and LIB$MULTF_DELTA_TIME to multiply delta times by scalar and floating scalar.

Table 27-2 lists all the LIB$ routines that perform date/time manipulation.

This section presents information about obtaining the current date and time, and setting current time. The run-time library (LIB$) facility provides date/time utility routines for languages that do not have built-in time and date functions. These routines return information about the current date and time or a date/time specified by the user. You can obtain the current time by using the LIB$DATE_TIME routine or by implementing the SYS$GETTIM system service. To set the current time, use the SYS$SETTIME system service.

Table 27-3 describes the date/time routines.

The LIB$DATE_TIME routine returns a character string containing the current date and time in absolute time format. The full string requires a declaration of 23 characters. If you specify a shorter string, the value is truncated. A declaration of 16 characters obtains only the date. The following example displays the current date and time:

! Formatted date and time
CHARACTER*23 DATETIME
! Status and library procedures
INTEGER*4 STATUS,
2         LIB$DATE_TIME
EXTERNAL  LIB$DATE_TIME
STATUS = LIB$DATE_TIME (DATETIME)
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
TYPE *, DATETIME

27.3.2 Obtaining Current Time and Date with SYS$GETTIM

/* Buffer to receive the binary time */
struct {
        unsigned int buff1, buff2;
}time;

   .
   .
   .
main() {

        unsigned status;

This call to SYS$GETTIM returns the current date and time in system format in the quadword buffer TIME.

The Convert Binary Time to ASCII String (SYS$ASCTIM) system service converts a time in system format to an ASCII string and returns the string in a 23-byte buffer. You call the SYS$ASCTIM system service as follows:

#include <stdio.h>
#include <descrip.h>

struct {
        unsigned int buff1, buff2;
}time_value;

main() {

        unsigned int status;
        char timestr[23];
        $DESCRIPTOR(atimenow, timestr);

/* Get binary time */
        status = SYS$GETTIM(&time_value);
        if ((status & 1) != 1)
                LIB$SIGNAL( status );

/* Convert binary time to  ASCII */
        status = SYS$ASCTIM(0,            /* timlen - Length of ASCII string */
                            &atimenow,    /* timbuf - ASCII time buffer */
                            &time_value,  /* timadr - Binary time */
                            0);           /* cvtflags - Conversion indicator */
        if ((status & 1) != 1)
                LIB$SIGNAL( status );


}

Because the address of a 64-bit time value is not supplied, the default value, 0, is used.

The string the service returns has the following format: