Cosmic time
In a different way, the Big Bang may be taken as reference to define  as the age of the universe, also known as time since the big bang. The current physical cosmology estimates the present age as 13.8 billion years. The  doesn't necessarily have to go along with a physical event (such as the cosmological singularity) but rather it refers to the point at which the scale factor would dissappear for a standard cosmological model such as Lambda-CDM model. For instance, in the case of inflation, In other words a non-standard cosmology, the hypothetical moment of big bang is still decided using the test result cosmological models which may happen at the same time as the end of the inflationary epoch. For technical purposes, ideas such as the average temperature of the universe (in units of eV) or the particle horizon are used when the early universe is the goal of a study since understanding the interaction among particles is more important and related than their time coordinate or age.