averageTime = (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). We'll refer 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 | + *5.84542x1011 | + *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 we do not want to use this as the normal type, because it is slow and does not
+ * have a fixed size.
+ *
+ *
+ * Because of these issues, we ended up concluding that the .NET framework's
+ * System.DateTime would be the best pivot. However, we use 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.
+ *
+ */
+
+/**
+ * UDateTimeScale values are used to specify the time scale used for
+ * conversion into or out if the universal time scale.
+ *
+ * @stable ICU 3.2
+ */
+typedef enum UDateTimeScale {
+ /**
+ * Used in the JDK. Data is a Java long (int64_t). Value
+ * is milliseconds since January 1, 1970.
+ *
+ * @stable ICU 3.2
+ */
+ UDTS_JAVA_TIME = 0,
+
+ /**
+ * Used on Unix systems. Data is int32_t or int64_t. Value
+ * is seconds since January 1, 1970.
+ *
+ * @stable ICU 3.2
+ */
+ UDTS_UNIX_TIME,
+
+ /**
+ * Used in IUC4C. Data is a double. Value
+ * is milliseconds since January 1, 1970.
+ *
+ * @stable ICU 3.2
+ */
+ UDTS_ICU4C_TIME,
+
+ /**
+ * Used in Windows for file times. Data is an int64_t. Value
+ * is ticks (1 tick == 100 nanoseconds) since January 1, 1601.
+ *
+ * @stable ICU 3.2
+ */
+ UDTS_WINDOWS_FILE_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.
+ *
+ * @stable ICU 3.2
+ */
+ UDTS_DOTNET_DATE_TIME,
+
+ /**
+ * Used in older Macintosh systems. Data is int32_t or int64_t. Value
+ * is seconds since January 1, 1904.
+ *
+ * @stable ICU 3.2
+ */
+ UDTS_MAC_OLD_TIME,
+
+ /**
+ * Used in newer Macintosh systems. Data is a double. Value
+ * is seconds since January 1, 2001.
+ *
+ * @stable ICU 3.2
+ */
+ UDTS_MAC_TIME,
+
+ /**
+ * Used in Excel. Data is an ?unknown?. Value
+ * is days since December 31, 1899.
+ *
+ * @stable ICU 3.2
+ */
+ UDTS_EXCEL_TIME,
+
+ /**
+ * Used in DB2. Data is an ?unknown?. Value
+ * is days since December 31, 1899.
+ *
+ * @stable ICU 3.2
+ */
+ UDTS_DB2_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).
+ *
+ * @stable ICU 3.8
+ */
+ UDTS_UNIX_MICROSECONDS_TIME,
+
+ /**
+ * The first unused time scale value. The limit of this enum
+ */
+ UDTS_MAX_SCALE
+} UDateTimeScale;
+
+/**
+ * UTimeScaleValue values are used to specify the time scale values
+ * to utmscale_getTimeScaleValue.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @stable ICU 3.2
+ */
+typedef enum UTimeScaleValue {
+ /**
+ * The constant used to select the units vale
+ * for a time scale.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @stable ICU 3.2
+ */
+ UTSV_UNITS_VALUE = 0,
+
+ /**
+ * The constant used to select the epoch offset value
+ * for a time scale.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @stable ICU 3.2
+ */
+ UTSV_EPOCH_OFFSET_VALUE=1,
+
+ /**
+ * The constant used to select the minimum from value
+ * for a time scale.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @stable ICU 3.2
+ */
+ UTSV_FROM_MIN_VALUE=2,
+
+ /**
+ * The constant used to select the maximum from value
+ * for a time scale.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @stable ICU 3.2
+ */
+ UTSV_FROM_MAX_VALUE=3,
+
+ /**
+ * The constant used to select the minimum to value
+ * for a time scale.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @stable ICU 3.2
+ */
+ UTSV_TO_MIN_VALUE=4,
+
+ /**
+ * The constant used to select the maximum to value
+ * for a time scale.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @stable ICU 3.2
+ */
+ UTSV_TO_MAX_VALUE=5,
+
+#ifndef U_HIDE_INTERNAL_API
+ /**
+ * The constant used to select the epoch plus one value
+ * for a time scale.
+ *
+ * NOTE: This is an internal value. DO NOT USE IT. May not
+ * actually be equal to the epoch offset value plus one.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @internal ICU 3.2
+ */
+ UTSV_EPOCH_OFFSET_PLUS_1_VALUE=6,
+
+ /**
+ * The constant used to select the epoch plus one value
+ * for a time scale.
+ *
+ * NOTE: This is an internal value. DO NOT USE IT. May not
+ * actually be equal to the epoch offset value plus one.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @internal ICU 3.2
+ */
+ UTSV_EPOCH_OFFSET_MINUS_1_VALUE=7,
+
+ /**
+ * The constant used to select the units round value
+ * for a time scale.
+ *
+ * NOTE: This is an internal value. DO NOT USE IT.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @internal ICU 3.2
+ */
+ UTSV_UNITS_ROUND_VALUE=8,
+
+ /**
+ * The constant used to select the minimum safe rounding value
+ * for a time scale.
+ *
+ * NOTE: This is an internal value. DO NOT USE IT.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @internal ICU 3.2
+ */
+ UTSV_MIN_ROUND_VALUE=9,
+
+ /**
+ * The constant used to select the maximum safe rounding value
+ * for a time scale.
+ *
+ * NOTE: This is an internal value. DO NOT USE IT.
+ *
+ * @see utmscale_getTimeScaleValue
+ *
+ * @internal ICU 3.2
+ */
+ UTSV_MAX_ROUND_VALUE=10,
+
+#endif /* U_HIDE_INTERNAL_API */
+
+ /**
+ * The number of time scale values, in other words limit of this enum.
+ *
+ * @see utmscale_getTimeScaleValue
+ */
+ UTSV_MAX_SCALE_VALUE=11
+
+} UTimeScaleValue;
+
+/**
+ * Get a value associated with a particular time scale.
+ *
+ * @param timeScale The time scale
+ * @param value A constant representing the value to get
+ * @param status The status code. Set to U_ILLEGAL_ARGUMENT_ERROR if arguments are invalid.
+ * @return - the value.
+ *
+ * @stable ICU 3.2
+ */
+U_STABLE int64_t U_EXPORT2
+ utmscale_getTimeScaleValue(UDateTimeScale timeScale, UTimeScaleValue value, UErrorCode *status);
+
+/* Conversion to 'universal time scale' */
+
+/**
+ * Convert a int64_t datetime from the given time scale to the universal time scale.
+ *
+ * @param otherTime The int64_t datetime
+ * @param timeScale The time scale to convert from
+ * @param status The status code. Set to U_ILLEGAL_ARGUMENT_ERROR if the conversion is out of range.
+ *
+ * @return The datetime converted to the universal time scale
+ *
+ * @stable ICU 3.2
+ */
+U_STABLE int64_t U_EXPORT2
+ utmscale_fromInt64(int64_t otherTime, UDateTimeScale timeScale, UErrorCode *status);
+
+/* Conversion from 'universal time scale' */
+
+/**
+ * Convert a datetime from the universal time scale to a int64_t in the given time scale.
+ *
+ * @param universalTime The datetime in the universal time scale
+ * @param timeScale The time scale to convert to
+ * @param status The status code. Set to U_ILLEGAL_ARGUMENT_ERROR if the conversion is out of range.
+ *
+ * @return The datetime converted to the given time scale
+ *
+ * @stable ICU 3.2
+ */
+U_STABLE int64_t U_EXPORT2
+ utmscale_toInt64(int64_t universalTime, UDateTimeScale timeScale, UErrorCode *status);
+
+#endif /* #if !UCONFIG_NO_FORMATTING */
+
+#endif
+