Dev Tools+: Cron, JWT, UUID and Text Diff

Dev Tools+ is the timing-and-text branch of Data & Dev: four panels that parse, decode or compare text. Cron parses five- and six-field expressions and reads them back as a schedule; JWT splits a token into header, payload and signature and flags expiry; UUID / ULID mints identifiers and reverse-reads timestamps from v1, v7 and ULID values; Text diff compares two texts at line, word or character granularity.

Updated 2026-10-01Reviewed by CalcX Editorial Team6 sources5 min read

How to use the panels

Cron.Enter an expression or pick a preset (@daily, 0 9-18 * * 1-5); read-back, field table and Upcoming runs update live. Copy or export 1–50 occurrences as .ics, or build one from the pickers.
JWT.Paste a token — a leading Bearer is tolerated — or load the RFC 7519 sample. Header and payload appear as JSON with alg, typ and kid badges, a status card, a time-claims table and claim lists; Show raw segments recolours the parts.
UUID / ULID.Pick v4, v7, v1 or ULID, set a count up to 500 and press Generate; Parse loads a row into the parser, which accepts braces, urn:uuid: and the unseparated form.
Text diff.Paste the two texts and choose line, word or char granularity; Ignore case and Ignore trailing WS fold differences away. Side by side, Inline and Unified diff change the view, with context lines and Copy.

Worked examples

Cron. */15 9-17 * * 1-5 reads "every 15 minutes during hour 9–17, on Monday–Friday"; from Monday 28 September 2026 10:30 the next four are 10:45, 11:00, 11:15, 11:30, and from Friday 2 October 17:50 the next is Monday 5 October 09:00. 0 0 13 * 5 reads "00:00, on day 13 of the month or on Friday", so from 1 October 2026 the list runs Fri 2 Oct, Fri 9 Oct, Tue 13 Oct, Fri 16 Oct. 0 0 29 2 * answers 2028-02-29; 30 0 3 * * * reads "03:00, at second 30, every day".
JWT. The RFC 7519 sample shows alg HS256, typ JWT, segments 40 / 94 / 43, a 32 B signature and claims iss = "joe", exp = 1300819380 and http://example.com/is_root. Status Expired: exp is 2011-03-22T18:43:00.000Z. A millisecond nbf (1700000000000) is corrected to 1700000000 and marked Not yet valid.
UUID / ULID. 018bcfe5-6800-7243-bd0c-de1cee9250a0 parses as UUID v7, 74 random bits, sortable, timestamp 2023-11-14T22:13:20.000Z; 01HF7YAT00NS2M9656TYVRKCYD as a ULID, 80 random bits, same instant — the two prefixes encode the same moment. 550e8400-e29b-41d4-a716-446655440000 is v4: 122 random bits, no timestamp.
Text diff. The built-in example reports distance 6 (+4 / −2), similarity 50.0 %, 5 → 7 lines, unified hunk @@ -1,5 +1,7 @@; one/two/three/four/five vs one/2/three/four/five/six gives distance 3, similarity 72.7 %; character mode turns "kitten" into "sitting" in 5 edits.

How the panels compute

Cron is the Vixie /etc/crontab dialect — names (JAN, MON), ranges, steps, macros and wrapping reverse ranges, with 7 as Sunday — matched in the local zone. JWT decoding is base64url only: five date claims are read as Unix seconds, and values above 10¹¹ are treated as milliseconds. Identifiers come from crypto.getRandomValues; UUID layouts follow RFC 9562 and ULIDs use Crockford Base32. The diff is Myers' O(ND).

Limits

Cron looks twelve years ahead.: When nothing falls inside the window the panel says "no occurrence" — the honest answer for 31 February.
Cron times are local.: A server's cron usually runs in UTC or the machine zone; convert before moving a schedule across.
JWT decodes, never trusts.: The signature is text plus a byte count; nothing is fetched, and a green status only means the time claims look sane.
Diff is capped at 20,000 tokens per side.: Character mode hits the cap first; longer inputs are refused with a suggestion to coarsen.
Not an encoding, network or security suite.: Base64, hex, hashes, JSON and regex live in Dev Tools; DNS and IP work in Network Tools and the IP subnet tool; arithmetic in the standard calculator.

Pitfalls

The two day fields are ORed, not ANDed. 0 0 13 * 5 fires every Friday and every 13th — not just Friday the 13th. The panel warns "it fires when either matches"; the AND belongs inside the job.
Adjacent deletes and inserts pair up. Side-by-side merges a removed line with the next added one into a modified row — the demo shows two modified rows and two inserts (blank line, export).
Word mode counts whitespace. "the quick brown fox" is seven tokens — four words, three spaces — so one swapped word is distance 2, not 1.

Where it is used

Checking a schedule before editing crontab

Read the expression back, count expanded values and cross a weekend in the upcoming list; .ics drops it into a calendar.

Reading a token from a bug report

Decode a quoted token without sending it anywhere — then treat the output as text: encoded payload, unchecked signature.

Choosing a key that sorts

Compare v4, v7 and ULID: v7 and ULID stay in insertion order as text; v4 hides creation time but scatters. Paste a value to recover its mint time.

Reviewing an edit before it ships

Switch granularity to match the question — line, word or char — and copy the patch.

References

Michael Kerrisk, crontab(5) — Linux manual page, man7.org （访问日期：2026-10-01）— field syntax and the OR rule.
IETF, RFC 7519: JSON Web Token (JWT), rfc-editor.org （访问日期：2026-10-01）— registered claims and the sample token.
IETF, RFC 7515: JSON Web Signature (JWS), rfc-editor.org （访问日期：2026-10-01）— compact serialization and the alg header.
IETF, RFC 9562: Universally Unique IDentifiers (UUID), rfc-editor.org （访问日期：2026-10-01）— v1, v4 and v7 layouts and variants.
ULID, Universally Unique Lexicographically Sortable Identifier specification, github.com/ulid （访问日期：2026-10-01）— Crockford Base32 layout and sort order.
Eugene W. Myers, An O(ND) Difference Algorithm and Its Variations, Algorithmica 1(2):251–266, 1986, link.springer.com （访问日期：2026-10-01）— the diff algorithm.