About

Raw millisecond values are ubiquitous in software logs, performance profiling, API response headers, and process.hrtime output. Misreading 1337000000ms as "about 13 million seconds" instead of the correct 15d 11h 23m 20s leads to incorrect timeout configurations, flawed SLA reports, and misdiagnosed latency bottlenecks. This converter decomposes any millisecond value - including sub-millisecond precision down to nanoseconds - into human-parseable duration strings using cascading integer division through time units from years (365.25 days) down to nanoseconds. It replicates the full API surface of the pretty-ms specification: compact mode, verbose labels, colon notation for digital-clock style, configurable decimal digits, unit count limits, and independent control over year, day, and second visibility. The tool assumes fixed-length days (86400s) and years (365.25d); it does not account for leap seconds or calendar irregularities because it formats durations, not calendar dates.

Formulas

The conversion decomposes a millisecond value t through cascading integer division across fixed time-unit boundaries:

years = t31557600000

days = t mod 3155760000086400000

hours = t mod 864000003600000

minutes = t mod 360000060000

seconds = t mod 600001000

ms = t mod 1000

Where the constants derive from: 1y = 365.25 × 24 × 3600 × 1000 = 31557600000ms. For sub-millisecond decomposition with formatSubMilliseconds enabled: μs = floor(fractional_ms × 1000) and ns = floor(fractional_μs × 1000). Each unit extraction uses floor division; the remainder cascades to the next smaller unit. The colonNotation mode formats as H:MM:SS with zero-padded minutes and seconds, always showing at least M:SS.

Reference Data

Millisecond Value	Default Output	Compact	Verbose	Colon Notation
500	500ms	500ms	500 milliseconds	0:00.5
1337	1.3s	1s	1.3 seconds	0:01.3
5000	5s	5s	5 seconds	0:05
60000	1m	1m	1 minute	1:00
95500	1m 35.5s	1m	1 minute 35.5 seconds	1:35.5
3600000	1h	1h	1 hour	1:00:00
86400000	1d	1d	1 day	24:00:00
1335669000	15d 11h 1m 9s	15d	15 days 11 hours 1 minute 9 seconds	371:01:09
31557600000	1y	1y	1 year	8766:00:00
100400080	1d 3h 53m 20s	1d	1 day 3 hours 53 minutes 20 seconds	27:53:20
0.0005	0ms	0ms	0 milliseconds	0:00
100.40008 (sub-ms)	100ms	100ms	100 milliseconds	0:00.1

Frequently Asked Questions

Duration formatting uses the Julian year convention of 365.25 days (31557600000ms) to account for leap years averaged over a four-year cycle. This matches the SI definition of the Julian year used in astronomical and scientific contexts. Calendar-specific year lengths (365 or 366 days) are irrelevant for duration math because durations measure elapsed time, not calendar spans.

JavaScript floating-point arithmetic (IEEE 754 double precision) introduces rounding artifacts below 1ms. The converter mitigates this by rounding intermediate microsecond and nanosecond values to the nearest integer after multiplication by 1000. Input values like 100.400080 yield 100ms 400μs 80ns when formatSubMilliseconds is enabled. Without that flag, the fractional milliseconds are silently truncated.

Compact mode (compact = TRUE) forces exactly one unit and zeroes all decimal digits, e.g., 1337000000ms becomes 15d. The unitCount option lets you specify any number of units to display, e.g., unitCount = 2 yields 15d 11h. Setting compact overrides unitCount.

Yes. Colon notation does not cap hours at 24. A value of 1335669000ms renders as 371:01:09, not as days plus hours. This mimics digital stopwatch behavior where the hour counter accumulates without rolling over into days.

With hideYear enabled, years are converted to their equivalent days (365.25 per year) and added to the day count: 1y 3d becomes 368d. With hideYearAndDays, both years and days collapse into hours: 1y 3d 5h becomes 8837h. This is useful for engineering contexts where only hours-level granularity matters.

By default, a value of exactly 13000ms shows as 13s (trailing .0 is stripped). Enabling keepDecimalsOnWholeSeconds preserves the decimal: 13.0s. The number of zeros shown matches secondsDecimalDigits. With secondsDecimalDigits = 3, it would show 13.000s. This prevents UI width jitter in progress displays.