feat: relative-time timestamp annotations + nil stdin-stat guard

AGENTS.md

@@ -47,7 +47,7 @@ Removing the fallback deleted the heuristics that existed only to decide it (`is
 ### `output.go` - Formatting, escaping, and colored printing
 
 - `printDecryptedPayload()` / `escapeTerminalText()` / `escapeFormattedJSONControls()` - Recursively decode nested JWTs/JWEs and pretty-print JSON objects or arrays; raw plaintext escapes C0 controls except newline/tab, DEL, C1 controls, invalid UTF-8 bytes, and targeted bidi controls, while formatted JSON sanitizes C1, DEL, and the same targeted bidi controls
-- `formatTimestamps()` / `timestampStatus()` - Convert exact `iat`, `exp`, `nbf` Unix numeric values, including fractions, to RFC3339 strings (original value shown in parentheses); `exp` in the past is annotated `expired` and `nbf` in the future `not yet valid`. This is display-only and never affects verification or the exit code; `timeNow` is a package variable so the annotations are testable. The `--json` path skips this entirely and keeps raw numeric claims
+- `formatTimestamps()` / `timestampStatus()` / `humanizeDuration()` - Convert exact `iat`, `exp`, `nbf` Unix numeric values, including fractions, to RFC3339 strings (original value shown in parentheses); `exp` is annotated with the time remaining or elapsed (`expires in 14m` / `expired 2h ago`) and a future `nbf` with `not yet valid, in 5m` (an already-valid `nbf` gets no note). `humanizeDuration` renders the largest whole unit (s/m/h/d), truncating toward zero for deterministic output. This is display-only and never affects verification or the exit code; `timeNow` is a package variable so the annotations are testable. The `--json` path skips this entirely and keeps raw numeric claims
 - `newFormatter()` - Creates a `go-prettyjson` formatter with the project color scheme
 - `printSection()` / `printSignature()` - Formatted output using `fatih/color`
 

README.md

@@ -16,7 +16,7 @@ A CLI tool that decodes and pretty-prints JSON Web Tokens (JWTs) and JSON Web En
 - `JWTD_KEY` environment variable for default key configuration
 - Syntax-highlighted JSON output with a consistent color scheme
 - Machine-readable output with `--json` for scripting and piping into tools like `jq`
-- Automatic conversion of `iat`, `exp`, and `nbf` timestamps to human-readable RFC3339 dates, with `expired` / `not yet valid` annotations
+- Automatic conversion of `iat`, `exp`, and `nbf` timestamps to human-readable RFC3339 dates, annotated with the time remaining or elapsed (`expires in 14m`, `expired 2h ago`, `not yet valid, in 5m`)
 - Accepts tokens as arguments, from stdin pipes, or via an interactive prompt
 - Colors auto-disable when output is not a TTY, or are controlled explicitly with `--color`
 - Shell completions (bash, zsh, fish) shipped in the Homebrew formula and the `.deb`/`.rpm` packages