"unixtime" unit difference computed incorrectly

if an item has "unixtime" units, difference for 30 seconds is shown as follows :

+1970.01.01 03:00:31

so the diff is shown as unix time stamp in absolute, not relative time (and adding the timezone shift - that's where the 3 hours come from).

for unixtime & uptime units diff should be calculated as time between those timestamps instead