Shorthand for
date &
time, mostly in programming languages. Why? Because (most) computers store the ever-changing
now as the number of seconds since the beginning of the
epoch.
The electronic bit-brain doesn't care about the orbits of a small ball of rock around a larger ball of dirt, which in turn orbits a huge mass of gas. So days, months, and years don't mean much to it.
The only reason that seconds matter to it, is that somebody hard wired a small device to its head, which says: "Has a second passed yet? OK, add 1 to my counter."
So whenever you want to know the date, or the time, the CPU asks the clock, and gives you back the number of seconds since the begining of the epoch (which is the dawn of time, in as much as it's concerned). The only reason you might see the date and time as days, monthes, years, minutes, hours and seconds is due to the work of a diligent programmer who calculates them for human consumption.