Using equation of motion:
The start of the motion determines the direction or convention.
Is the object thrown upwards? What is the objects initial location?
The first information tell use the convention for velocity which displacement and acceleration must follow. If throwing up has a positive velocity ,acceleration up means positive , then acceleration = -9.81ms-2 and displacement above the starting point (s=0)is positive and below it is negative.The quantity time has no direction.
