A Practical Guide to Time for Developers: Part 1 — What time is in software (physics + agreements)

Preface to the series I was tasked with synchronizing time across N computers with ~1 nanosecond accuracy . Not “a laptop over Wi-Fi” — a controlled wired setup where hardware timestamping and disciplined clocks make that goal at least a meaningful engineering target. At first it sounded trivial. We learn clocks, dates, and time zones as kids. How hard can it be? The industry already has a standard solution: Precision Time Protocol (PTP) . But I wanted to look inside the protocol and understand what it actually does. I expected it to be the easiest part of the whole story. Instead I ran straight into a wall of concepts: TAI vs UTC, epochs, leap seconds, RTC vs system clock, wall clock vs monotonic time, time zones, naïve timestamps . It turns out “time” is not a single thing — it’s physics, standards, and human conventions layered on top of each other. I searched for a single article that explains the whole chain — something like “Time for software developers: zero to hero” or “From RT