Let's then introduce a grid level directly below the measure but above the beat. I'll call this the beat group. The idea here is that the 4 beats of a 4/4 measure can be thought of as two groups of two. Similarly, something like 5/8 can be thought of as a two-beat group followed by a three-beat group or vice versa.
We'll say that the time-offset of the g-th beat group of the m-th measure from the start of the measure is Tmg.
Hence the the absolute time offset of the g-th beat group of the m-th measure would be Tm + Tmg.