Timestamp Batch Converter

The unit problem, at scale

Working through a list makes the seconds-versus-milliseconds confusion visible immediately: a value read in the wrong unit lands either in 1970 or tens of thousands of years out. A present-day timestamp is 10 digits in seconds and 13 in milliseconds, so magnitude detection is reliable. Where a list mixes both, which happens when data is merged from two systems, detecting per row rather than per file is the only correct approach.

Choose the display timezone deliberately

A timestamp is an absolute instant with no timezone. Rendering it requires choosing one, and the default is whatever the viewer's machine is set to. That means the same log file produces different-looking output for two colleagues, which turns an incident timeline into an argument. For anything shared, render in UTC and say so.

Daylight saving makes some local times ambiguous

In the hour that repeats when clocks go back, a local time maps to two distinct instants. In the hour that is skipped when they go forward, some local times do not exist at all. Converting a list of local times to timestamps therefore has no unique answer for those rows, which is a genuine reason to store and exchange UTC rather than local time.

Common formats worth recognising

Unix seconds and milliseconds are the usual pair. Microseconds appear in database exports. Windows FILETIME counts 100-nanosecond intervals since 1601 and produces very large numbers. Excel serial dates count days since 1900, with the deliberate leap-year bug retained for compatibility. Recognising which you have from the magnitude saves a lot of confusion.

Batch work benefits from a sanity check

Convert the first row and the last row and check they are plausible before trusting the middle. A whole file converted with the wrong unit assumption looks internally consistent and is uniformly wrong, which is far harder to notice than a single bad value.

Processed locally

Conversion happens in your browser. Log extracts frequently contain user identifiers and internal hostnames, so the data staying on your machine matters.