Last System Restart information from application level

 If you want to check the last SAP system (Application Server) Restart time, we can easily find it from ST02 Transaction.

Goto ST02 ----> Check the Startup time on the top

If you want to find the DB Startup time check Transaction ST04

Or DB02 ---> Performance ---> Performance Overview

Also, you will get the information from

AL11 ---> DIR_HOME (/usr/sap/SID/DVEBMGSXX/work)

Check available.log 

This data will get from the same directory on the OS level.