Epoch & Unix Timestamp Converter

What is Unix time?

Unix time (epoch time) counts seconds since 00:00:00 UTC on January 1, 1970, the "Unix epoch," ignoring leap seconds. It's timezone-free by design: the same number means the same instant everywhere, which is why it's the default way APIs, databases and token formats like JWT exchange points in time.

Seconds vs. milliseconds vs. microseconds vs. nanoseconds

Most systems use whole seconds (a 10-digit number today), but JavaScript's Date.now() and many logging/tracing systems use milliseconds (13 digits), and some high-resolution systems use microseconds or nanoseconds. This tool detects the unit from the number of digits and lets you override it if the auto-detection guesses wrong.

What is the Year 2038 problem?

Systems that store Unix time in a signed 32-bit integer will overflow at 03:14:07 UTC on January 19, 2038, wrapping around to a date in 1901. Modern 64-bit systems (including .NET's DateTimeOffset and JavaScript's number) aren't affected, but it still matters for older databases, file formats and embedded systems.

exp, nbf, iat and auth_time in JWTs

JWTs encode expiration (exp), not-before (nbf), issued-at (iat) and, for ID tokens, authentication time (auth_time) as Unix timestamps in seconds. See the JWT decoder for a full breakdown of a token's claims, or paste just the time claims into the helper above. Learn more in managing token expiration, refresh & revocation in web APIs.

What is clock skew?

Because server clocks are never perfectly synchronized, token validation libraries tolerate a small amount of clock skew when checking exp and nbf. Both IdentityServer and ASP.NET Core's JWT Bearer handler default ClockSkew to 5 minutes. See token handling in IdentityServer for how to tune it.

Epoch conversion in .NET

.NET's DateTimeOffset has built-in Unix time conversions, which avoid manual math and timezone bugs:

// Unix seconds -> DateTimeOffset
var when = DateTimeOffset.FromUnixTimeSeconds(1700000000L);

// DateTimeOffset -> Unix seconds
long epoch = DateTimeOffset.UtcNow.ToUnixTimeSeconds();

// Milliseconds variants also exist:
DateTimeOffset.FromUnixTimeMilliseconds(1700000000000L);
DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();