XPLM_API float XPLMGetElapsedTime(void);
This routine returns the elapsed time since the sim started up in decimal seconds. This is a wall timer; it keeps counting upward even if the sim is pasued.
WARNING: XPLMGetElapsedTime is not a very good timer! It lacks precision in both its data type and its source. Do not attempt to use it for timing critical applications like network multiplayer.