don't think there is a special field for this in the heap dump.
If there is any object keeping the startup date/time one could see it. I had a look at the System properties, but I didn't find there such a property.
One thing that could work:
- when starting the VM you pass an additional system property, keeping the start date/time, i.e. something like -DstartTime=<time>. You can pass date and time from the OS commands
- once you trigger a heap dump and open it in MAT, you open from the query menu "Java Basics" -> "System properties" and see the start date/time
- in the heap dump details view you can see the time when the heap dump is taken
It's not diretly the value "uptime" but you can calcualte it.
Let me know if this helps.