Both of you are right in slightly different senses.
There is no "global time clock" and hence in a philosophical sense there is no "common now".
However, the speed of light is constant in the universe and so you can calculate time based on distance traveled by light to you provided you are not moving at speeds comparable to speed of light. This means you can suppose a "common lightyear clock" for measurement.
But when the speeds are comparable to that of light, you have time dilation and length contraction and so even the "common lightyear clock" is invalid. This is the principle of "Relativity of Simultaneity".
The way to think about it is, Events happen in a Space-Time fabric and so in order to measure the interval between two events you have to consider both Space and Time w.r.t. object speed vs. speed of light. The universal invariant here is the "Space-Time Interval" between two events which will be the same for everybody whatever be their mass or speed.
There is no "global time clock" and hence in a philosophical sense there is no "common now".
However, the speed of light is constant in the universe and so you can calculate time based on distance traveled by light to you provided you are not moving at speeds comparable to speed of light. This means you can suppose a "common lightyear clock" for measurement.
But when the speeds are comparable to that of light, you have time dilation and length contraction and so even the "common lightyear clock" is invalid. This is the principle of "Relativity of Simultaneity".
The way to think about it is, Events happen in a Space-Time fabric and so in order to measure the interval between two events you have to consider both Space and Time w.r.t. object speed vs. speed of light. The universal invariant here is the "Space-Time Interval" between two events which will be the same for everybody whatever be their mass or speed.