Returns the number of milliseconds passed since the device was last reset. This function is similar to the global
millis() function but returns a 64-bit value.
While the 32-bit
millis() rolls over to 0 after approximately 49 days, the 64-bit
System.millis() does not.
One caveat is that sprintf-style formatting, including
String::format() etc. does not support 64-bit integers. It does not support
%llu or Microsoft-style
As a workaround you can use the
Print64 firmware library in the community libraries. The source and instructions can be found in Github.