Problem description: the following runtime error is observed when a user previews or opens is view mode (in Monitoring => Hosts => Graphs ) a graph that has some specific item (for example the Host local time item) with "Y axis MAX value" set to Calculated and that doesn't have any values to show yet:

Exception: DateTime::__construct(): Failed to parse time string (@0.2) at position 3 (2): Unexpected character in /home/sergejs/workspace/6.0/ui/include/func.inc.php:205
Stack trace:
#0 /home/sergejs/workspace/6.0/ui/include/func.inc.php(205): DateTime->__construct()
#1 /home/sergejs/workspace/6.0/ui/include/func.inc.php(689): zbx_date2str()
#2 /home/sergejs/workspace/6.0/ui/include/func.inc.php(631): convertUnitsRaw()
#3 /home/sergejs/workspace/6.0/ui/include/graphs.inc.php(965): convertUnits()
#4 /home/sergejs/workspace/6.0/ui/include/classes/graphdraw/CLineGraphDraw.php(1172): calculateGraphScaleValues()
#5 /home/sergejs/workspace/6.0/ui/include/classes/graphdraw/CLineGraphDraw.php(2093): CLineGraphDraw->drawVerticalScale()
#6 /home/sergejs/workspace/6.0/ui/chart3.php(194): CLineGraphDraw->draw()
#7 {main}

Steps to reproduce:

1. Make sure Zabbix server isn't running
2. Create a host with the Host local time item
3. Navigate to Graphs section for this host and press "Create graph" button
4. In the opened form select the Host local time item and set "Y axis MAX value" set to "Calculated".
5. Switch to the "Preview" tab

Result: the above runtime error is observed, the graph is not loaded. This runtime error is also observed when opening the graph in Monitoring => Hosts => Graphs view
Expected: no runtime errors should be observed, empty graph should be displayed