It is little difficult to tell because Let us assume a RAC have Two nodes. Instance running on Node1 is failed and restarted on Monday but Instance running on Node2 is active. So if you look at GV$INSTANCE it will tell NODE1 Startup Time is Monday and NOD2 Startup time is different. On the other day if instance on Node2 went down and restarted, but Instance on Node1 is up and running. So if you look at now in GV$INSTANCE will say Node started on Monday and Node2 started on Tuesday but actually database was not down for long time.
So you have to check DBA_HIST_DATABASE_INSTANCE to know the actual Database Uptime

GV$INSTANCE – Instance Name and Startup Time

DBA_HIST_DATABASE_INSTANCE – Instance Start/Stop History