RTC за время сеанса используется только один раз - при загрузке системы время из него считывается и устанавливается системное время. После этого системные часы и RTC идут совершенно независимо.

In this mode, the system may adjust the time of day using its own internal time synchronization mechanisms. These internal time synchronization mechanisms may cause the time-of-day clock to change during the normal course of the system operation, which can include noticeable jumps in time as deemed necessary by the system.

надеяться, что БИОС достаёт время из RTC, а то мало ли.

BOOL WINAPI GetSystemTimeAdjustment(
_Out_ PDWORD lpTimeAdjustment,
_Out_ PDWORD lpTimeIncrement,
_Out_ PBOOL lpTimeAdjustmentDisabled
);

Parameters
lpTimeAdjustment [out]

A pointer to a variable that the function sets to the number of lpTimeIncrement100-nanosecond units added to the time-of-day clock for every period of time which actually passes as counted by the system. This value only has meaning if lpTimeAdjustmentDisabled is FALSE.
lpTimeIncrement [out]

A pointer to a variable that the function sets to the interval in 100-nanosecond units at which the system will add lpTimeAdjustment to the time-of-day clock. This value only has meaning if lpTimeAdjustmentDisabled is FALSE.
lpTimeAdjustmentDisabled [out]

A pointer to a variable that the function sets to indicate whether periodic time adjustment is in effect.

A value of TRUE indicates that periodic time adjustment is disabled, and the system time-of-day clock advances at the normal rate. In this mode, the system may adjust the time of day using its own internal time synchronization mechanisms. These internal time synchronization mechanisms may cause the time-of-day clock to change during the normal course of the system operation, which can include noticeable jumps in time as deemed necessary by the system.

A value of FALSE indicates that periodic time adjustment is being used to adjust the time-of-day clock. For each lpTimeIncrement period of time that actually passes, lpTimeAdjustment will be added to the time of day. If the lpTimeAdjustment value is smaller than lpTimeIncrement, the system time-of-day clock will advance at a rate slower than normal. If the lpTimeAdjustment value is larger than lpTimeIncrement, the time-of-day clock will advance at a rate faster than normal. If lpTimeAdjustment equals lpTimeIncrement, the time-of-day clock will advance at its normal speed. The lpTimeAdjustment value can be set by calling SetSystemTimeAdjustment. The lpTimeIncrement value is fixed by the system upon start, and does not change during system operation. In this mode, the system will not interfere with the time adjustment scheme, and will not attempt to synchronize time of day on its own via other techniques.

Ага, а что будет, если вызвать setsystemtimeadjustment с lpTimeAdjustmentDisabled, установленным в true? По идее, система должна вернуться к интервалам по умолчанию, и снова перевести свои "часы" в "режим отставания". На самом деле, ничего такого не происходит, "часы" продолжают тикать относительно точно, хотя GetSystemTimeAdjustment возращает как и должен 157250,156250,true.

на дефолт сбрасывается, когда батарейке совсем каюк, а когда она подсевшая то начинается вот такое с временем

Пока только в Windows время не правильно, в биосе надо будет проверить.

Посмотри в биосе, оно там уходит, или только в винде?