The push-pull transmission of electrical power, say ±5 volts, between a computer's memory and a serial device such as a keyboard, a computer screen, or a telephone modem, occurs by synchronizing the clock speed of the computer and the device, and then, in a single clock cycle, a negative voltage signal equals a "1" or a single digital computer bit. If this negative voltage remains constant for two clock cycles that would be "11".
If it switches to a positive voltage for four clock cycles that would be "0000". Back to a negative voltage for two clock cycles for "11".
Thus the negative or positive voltage signals, combined with the clock speed, enables the transmission of 8 bits as "11000011" , which translates into a single byte or character. Quite similar to sending (and receiving) Morse code, except the "byte" size is fixed. A collection of bytes — say 1500 bytes or so — then becomes a packet which is converted by a modem into a continuous analog signal over the Internet. Each packet contains a header section. The header contains the destination IP address and the source IP address which enables switching i.e. forwarding through individual routers to a destination and then a reply sent back to the source. Click here for further Internet information.
Now back to digital signals and clock cycles. A computer’s clock speed is determined by a quartz crystal which oscillates (vibrates) at a certain frequency and is usually expressed in MHz (mega-hertz, million of cycles/second) or GHz (giga-hertz, billion of cycles/second). Most computer microprocessors execute one or more basic instructions in memory, like adding two numbers together, per clock cycle. A 32-bit microprocessor has a group of 32 wires, called a data path or a parallel bus, and transmits 32 bits along this computer bus - from memory - through the processor and its ALU (arithmetic/logic unit) and back to memory with each instruction. And in this scenario, 3.3 volts on a wire is a "1" and zero volts on a wire is a "0".
Click here to read a great article on the German chap who discovered this brilliant computer "Clock Cycle" principle.
And click here to read about the convoluted process over thousands of years, under no one person or group's control, by which today's clock/calendar came into being, culminating in its use worldwide over the past 100 years.