Comment 2 for bug 1780341

Squid logs are a customized timestamp format based on UNIX times but also including millisecond (and optionally nanosecond) resolution not available in strictly UNIX timestamps.

This format and resolution is necessary for accuracy of log information - which is intended more for machine interpretation via log analysis tools, not human.

Human timestamps are far to inaccurate and malleable for this type of data recording. They also vary depending on where in the world the reader is located, independent of where the proxy is located. Human calendar systems are a legal fiction, not a technical or scientific measure - the government of any country can redefine them at any time (and some do so even today). So the time of the recording matters as much as the value recorded in human systems.
 Mapping between the fixed and deterministic Squid log values and a readers local human system is the purpose of log analysis tools and far more easily done from UNIX-like timestamps than other types.

For admin who wish to read the logs manually and provide inaccurate data to their analysis tools other formats are configurable.