datetime: fix negative nsec handling

Seems that problem code part was ported from Lua as is. But there
is some difference between modulo operator in C and in Lua. Lua
always returns positive value but in C result could be negavive.
This difference led to the case when after subtraction nsec part
of datetime object become negative that yielded weird result on
attempt to get string representation (e.g."2008-02-03T03:36:43.-100Z").
This patch fixes it.

Part of #8570

NO_DOC=bugfix
NO_CHANGELOG=see next commit