Tizen Native API
7.0

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:
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  5.84542x10^{11}  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
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:
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:
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.