Please check the Help for the default timer mode argument:
Returns the current date from the system clock. The timer_mode may have the following values:
0 Date at script run
1 Date at function call
2 Date when the document was opened
Default timer_mode is 2. The timer_mode = 1 should be used with caution, since it polls the operating system every second and hence could slow down the system.
So, if you are using timer_mode 2 (default), it is indeed only updated when you open the document.