Let's then say that the time-offset of the m-th measure is Tm.
This remains true.
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 (sic) absolute time offset of the g-th beat group of the m-th measure would be Tm + Tmg.
We'll now 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 absolute time offset of the g-th beat group of the m-th measure would be Tmg = Tm + tmg.