1. Maybe you can accomplish this, using report to see last user logoff/logon time on managed client PC?
2. There is a view "vRebootHistory" but it contains data only from Servers where Inventory Agent and Server Inventory Pack are installed.
3. If you want to know exact client PC reboot date, then you can create client script task (with enabled 'save task output' option) to get it via powershell or cmd script
Cmd example:
systeminfo | find /i "Boot Time"
PowerShell:
Get-WmiObject -class Win32_OperatingSystem | Select-Object __SERVER,@{label='LastBootUpTime';expression={$_.ConvertToDateTime($_.LastBootUpTime)}}
After task execution, every record will be populated in "TaskOutputPropertyValue" sql table for every new TaskInstance(s). Create custom SQL report to get appropriate data from this SQL table or use existing "tmSelectTaskOutputProperties" stored procedure and specify there appropriate Task Instance guid to get task output.