Universal Time Scale.

Required Header

#include <utils_i18n.h>

Overview

There are quite a few different conventions for binary datetime, depending on different platforms and protocols. Some of these have severe drawbacks. For example, people using Unix time (seconds since Jan 1, 1970) think that they are safe until near the year 2038. But cases can and do arise where arithmetic manipulations causes serious problems. Consider the computation of the average of two datetimes, for example: if one calculates them with average_time = (time1 + time2)/2, there will be overflow even with dates around the present. Moreover, even if these problems don't occur, there is the issue of conversion back and forth between different systems.

Binary datetimes differ in a number of ways: the datatype, the unit, and the epoch (origin). ICU refers to these as time scales. For example:

Table 1: Binary Time Scales
Source	Datatype	Unit	Epoch
UDTS_JAVA_TIME	int64_t	milliseconds	Jan 1, 1970
UDTS_UNIX_TIME	int32_t or int64_t	seconds	Jan 1, 1970
UDTS_ICU4C_TIME	double	milliseconds	Jan 1, 1970
UDTS_WINDOWS_FILE_TIME	int64_t	ticks (100 nanoseconds)	Jan 1, 1601
UDTS_DOTNET_DATE_TIME	int64_t	ticks (100 nanoseconds)	Jan 1, 0001
UDTS_MAC_OLD_TIME	int32_t or int64_t	seconds	Jan 1, 1904
UDTS_MAC_TIME	double	seconds	Jan 1, 2001
UDTS_EXCEL_TIME	?	days	Dec 31, 1899
UDTS_DB2_TIME	?	days	Dec 31, 1899
UDTS_UNIX_MICROSECONDS_TIME	int64_t	microseconds	Jan 1, 1970

All of the epochs start at 00:00 am (the earliest possible time on the day in question), and are assumed to be UTC.

The ranges for different datatypes are given in the following table (all values in years). The range of years includes the entire range expressible with positive and negative values of the datatype. The range of years for double is the range that would be allowed without losing precision to the corresponding unit.

Units	int64_t	double	int32_t
1 sec	5.84542x10¹¹	285,420,920.94	136.10
1 millisecond	584,542,046.09	285,420.92	0.14
1 microsecond	584,542.05	285.42	0.00
100 nanoseconds (tick)	58,454.20	28.54	0.00
1 nanosecond	584.5420461	0.2854	0.00

These functions implement a universal time scale which can be used as a 'pivot', and provide conversion functions to and from all other major time scales. This datetimes to be converted to the pivot time, safely manipulated, and converted back to any other datetime time scale.

So what to use for this pivot? Java time has plenty of range, but cannot represent .NET System.DateTime values without severe loss of precision. ICU4C time addresses this by using a double that is otherwise equivalent to the Java time. However, there are disadvantages with doubles. They provide for much more graceful degradation in arithmetic operations. But they only have 53 bits of accuracy, which means that they will lose precision when converting back and forth to ticks. What would really be nice would be a long double (80 bits -- 64 bit mantissa), but that is not supported on most systems.

The Unix extended time uses a structure with two components: time in seconds and a fractional field (microseconds). However, this is clumsy, slow, and prone to error (you always have to keep track of overflow and underflow in the fractional field). BigDecimal would allow for arbitrary precision and arbitrary range, but the ICU team did not want to use this as the normal type, because it is slow and does not have a fixed size.

Because of these issues, the ICU team ended up concluding that the .NET framework's System.DateTime would be the best pivot. However, ICU uses the full range allowed by the datatype, allowing for datetimes back to 29,000 BC and up to 29,000 AD. This time scale is very fine grained, does not lose precision, and covers a range that will meet almost all requirements. It will not handle the range that Java times do, but frankly, being able to handle dates before 29,000 BC or after 29,000 AD is of very limited interest.

Functions
int	i18n_utmscale_get_time_scale_value (i18n_utmscale_scale_e time_scale, i18n_utmscale_value_e utmscale_val, int64_t *value)
	Gets a value associated with a particular time scale.
int	i18n_utmscale_from_int64 (int64_t other_time, i18n_utmscale_scale_e time_scale, int64_t *universal_time)
	Converts the time given as an int64_t to Universal Time Scale.
int	i18n_utmscale_to_int64 (int64_t universal_time, i18n_utmscale_scale_e time_scale, int64_t *other_time)
	Converts the time given as Universal Time Scale to an int64_t.

Enumeration Type Documentation

enum i18n_utmscale_scale_e

Enumeration for the values used to specify the time scale used for conversion into or out of the universal time scale.

Since :: 3.0

Enumerator:

I18N_UTMSCALE_JAVA_TIME	Used in the JDK. Data is a Java long (int64_t). Value is milliseconds since January 1, 1970.
I18N_UTMSCALE_UNIX_TIME	Used on Unix systems. Data is an int32_t or int64_t. Value is seconds since January 1, 1970.
I18N_UTMSCALE_ICU4C_TIME	Used in ICU4C. Data is a double. Value is milliseconds since January 1, 1970.
I18N_UTMSCALE_WINDOWS_FILE_TIME	Used in Windows for file times. Data is an int64_t. Value is ticks (1 tick == 100 nanoseconds) since January 1, 1601.
I18N_UTMSCALE_DOTNET_DATE_TIME	Used in the .NET framework's System.DateTime structure. Data is an int64_t. Value is ticks (1 tick == 100 nanoseconds) since January 1, 0001.
I18N_UTMSCALE_MAC_OLD_TIME	Used in older Macintosh systems. Data is an int32_t or int64_t. Value is seconds since January 1, 1904.
I18N_UTMSCALE_MAC_TIME	Used in newer Macintosh systems. Data is a double. Value is seconds since January 1, 2001.
I18N_UTMSCALE_EXCEL_TIME	Used in Excel. Value is days since December 31, 1899.
I18N_UTMSCALE_DB2_TIME	Used in DB2. Value is days since December 31, 1899.
I18N_UTMSCALE_UNIX_MICROSECONDS_TIME	Data is a long. Value is microseconds since January 1, 1970. Similar to Unix time (linear value from 1970) and struct timeval (microseconds resolution).
I18N_UTMSCALE_MAX_SCALE	The first unused time scale value. The limit of this enum

enum i18n_utmscale_value_e

Enumeration for the values used to specify the time scale values to i18n_utmscale_get_time_scale_value().

Since :: 3.0

See also:: i18n_utmscale_get_time_scale_value()

Enumerator:

I18N_UTMSCALE_VALUE_UNITS	The constant used to select the units value for a time scale.
I18N_UTMSCALE_VALUE_EPOCH_OFFSET	The constant used to select the epoch offset value for a time scale.
I18N_UTMSCALE_VALUE_FROM_MIN	The constant used to select the minimum from value for a time scale.
I18N_UTMSCALE_VALUE_FROM_MAX	The constant used to select the maximum from value for a time scale.
I18N_UTMSCALE_VALUE_TO_MIN	The constant used to select the minimum to value for a time scale.
I18N_UTMSCALE_VALUE_TO_MAX	The constant used to select the maximum to value for a time scale.
I18N_UTMSCALE_VALUE_MAX	The number of time scale values, in other words limit of this enum.

Function Documentation

int i18n_utmscale_from_int64	(	int64_t	other_time,
		i18n_utmscale_scale_e	time_scale,
		int64_t *	universal_time
	)

Converts the time given as an int64_t to Universal Time Scale.

Since :: 3.0

Parameters:

[in]	other_time	The int64_t datetime
[in]	time_scale	The time scale to convert from
[out]	universal_time	The datetime converted to the universal time scale

Returns:: 0 on success, otherwise a negative error value

Return values:

I18N_ERROR_NONE	Successful
I18N_ERROR_INVALID_PARAMETER	The conversion is out of range.

int i18n_utmscale_get_time_scale_value	(	i18n_utmscale_scale_e	time_scale,
		i18n_utmscale_value_e	utmscale_val,
		int64_t *	value
	)

Gets a value associated with a particular time scale.

Since :: 3.0

Parameters:

[in]	time_scale	The time scale
[in]	utmscale_val	A constant representing the value to get
[out]	value	The obtained value

Returns:: 0 on success, otherwise a negative error value

Return values:

I18N_ERROR_NONE	Successful
I18N_ERROR_INVALID_PARAMETER	Invalid function parameter

int i18n_utmscale_to_int64	(	int64_t	universal_time,
		i18n_utmscale_scale_e	time_scale,
		int64_t *	other_time
	)

Converts the time given as Universal Time Scale to an int64_t.

Since :: 3.0

Parameters:

[in]	universal_time	The datetime in the Universal Time Scale
[in]	time_scale	The time scale to convert to
[out]	other_time	The datetime converted to the given time scale

Returns:: 0 on success, otherwise a negative error value

Return values:

I18N_ERROR_NONE	Successful
I18N_ERROR_INVALID_PARAMETER	The conversion is out of range.

Required Header

Overview

Functions

Enumeration Type Documentation

Function Documentation