How do you set the time resolution in Synplify?
(Arguably this is not an answer, more a "confirmation of the question" using the full version of the tool). I'll delete it if people would rather...



The reference manual for my version of Synplify Pro (I-2014.03-SP1) has this statement in it:


  The support of predefined physical time types includes the expanded
  range from –2147483647 to +2147483647 with units ranging from
  femtoseconds, and secondary units ranging up to an hour. Predefined
  physical time types allow selection of a wide number range
  representative of time type.


And some example code...

 entity test is
    generic (INTERVAL1 : time := 1000 fs;
    INTERVAL2 : time := 1 ps;
    INTERVAL3 : time := 1000 ps;
    INTERVAL4 : time := 1 ns
    );
 -- etc - comparing those values


is given.  This implies it's possible!

However, when trying a testcase based on said code it reports 


  @N: CD720 :"C:\Synopsys\fpga_I-2014.03-SP1\lib\vhd\std.vhd":123:18:123:21|Setting time resolution to ns


And a very simple testcase fails this assertion:

assert 1 ns = 1000 ps severity failure;


Which matches the sort of thing you're seeing.

And I can't see how to change the resolution anywhere.