What Errors Arise From Misreading Commcan Millis Hours?

Alright, quick and blunt take: misreading millis vs hours in comms/CAN gives you four big families of problems — overflow/precision errors, wrong scheduling (too fast or too slow), broken timestamp alignment across devices, and misleading logs that wreck troubleshooting. For example, using 16-bit ints for millisecond counters overflows in about a minute; treating milliseconds as hours makes intervals trillions of times off; integer truncation on division hides sub-hour activity; and non-wrap-safe comparisons break after a timer wrap. Simple mitigations I use: clear variable names that include units, central conversion macros/constants, use 64-bit where needed, and always do time checks like (now - last) >= interval to survive wraparound. If you’re on a CAN bus, standardize the epoch or include unit info in your frames — syncing clocks (or at least agreeing on units) saves a ton of pain. Honestly, it’s one of those dumb mistakes that’s easy to prevent and notoriously annoying to debug, so document time units aggressively and you’ll sleep better.

Man, mixing up milliseconds and hours in comms or CAN timestamps is one of those tiny blunders that sneaks up and then nukes your debugging session for half a day. When code thinks a value is in ms but the source is actually in hours (or vice versa), you get timing catastrophes: timeouts that never trigger, spurious retransmits, logs that look like the system slept through a month, and scheduling that fires either a million times or not at all. On embedded platforms like Arduino, millis() is an unsigned 32-bit counter that wraps about every 49.7 days — if somebody stores it in a 16-bit value by mistake, it overflows in ~65 seconds and everything goes bonkers. Similarly, integer division when converting milliseconds to hours (dividing by 3,600,000) will truncate toward zero, so tiny elapsed times become zero hours until a full hour has passed, which can hide errors or delay actions for way longer than intended.

Beyond these arithmetic slip-ups, protocol-level misreads are brutal in multi-node systems. If one ECU stamps CAN frames in milliseconds and another logs in seconds or hours, correlation across nodes becomes impossible: event ordering is scrambled, diagnostics look contradictory, and replay/forensics fail. Time comparisons are another common trap — comparing timestamps directly instead of using wrap-safe subtraction (current - previous >= interval) means you’ll mis-handle the wraparound case and either skip events or treat old events as fresh. Then there’s the behavioral fallout: if a watchdog expects heartbeat intervals in seconds but receives values thought to be hours, the watchdog might never reset and the entire system could stay in a failed state. In safety-critical gear, that’s more than annoying; it’s dangerous.

Fixes I’ve leaned on: always name variables with units, e.g., 'lastHeartbeatMs' or 'uptimeHours'; use appropriately sized types (64-bit if you need multi-year counters); perform wrap-safe comparisons; centralize time conversion constants (const unsigned long MS_PER_HOUR = 3600000UL;) and avoid ad-hoc divides sprinkled through the code. For CAN networks, pick one canonical timebase or embed unit metadata in the protocol. And log both the raw tick value and a human-friendly timestamp when debugging — that saved me on a week-long hunt once. Bottom line: unit mismatches feel like minor typos but they cascade into protocol errors, overflow bugs, silent failures, and very confusing logs, so treat time units with the same respect you give memory allocation and concurrency.