ADDITIONAL SYSTEM INFORMATION :
Mac OS 11.4

A DESCRIPTION OF THE PROBLEM :
Duration.parse() parse 'PT-0.999S' to 'PT0.999S',because Long.parseLong() method parse '-0' and '0' to same result 0 .

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
 String text = "PT-0.999S";
 Duration d = Duration.parse(text);
 assert d.toString().equals(text);

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
no error
ACTUAL -
assert error

---------- BEGIN SOURCE ----------
 String text = "PT-0.999S";
 Duration d = Duration.parse(text);
 assert d.toString().equals(text);
---------- END SOURCE ----------

FREQUENCY : always