Time & Time ZonesThe rules that prevent the entire category of date bugs.ComputingDates are the most consistently underestimated source of bugs in software.AlwaysStore timestamps in UTCStore the IANA zone name, not the offsetUse ISO 8601: 2026-09-05T14:30:00ZUse the platform's date libraryNever assumeA day is 24 hours — DST breaks itOffsets are whole hours — some are :30 or :45A zone's rules are fixed — they change by lawA year has 365 days, or a minute 60 secondsGotchas2 a.m. can occur twice, or not at allSorting local times across zones is wrong'Tomorrow' depends on the user's zoneUnix time ignores leap secondsAn offset like +01:00 is not a time zone. Europe/London is — becauseonly the name knows when the rules change.Time & Time Zoneslearnposters.com
Time & Time Zones — printable computing wall chart from LearnPosters. Free vector PDF, US Letter and A4.

What’s on the Time & Time Zones poster

12 entries in 3 groups.

Dates are the most consistently underestimated source of bugs in software.

An offset like +01:00 is not a time zone. Europe/London is — because only the name knows when the rules change.

Questions about the Time & Time Zones poster

What’s on the Time & Time Zones poster?
12 entries in 3 groups. Dates are the most consistently underestimated source of bugs in software. Always: Store timestamps in UTC, Store the IANA zone name, not the offset, Use ISO 86…; Never assume: A day is 24 hours — DST breaks it, Offsets are whole hours — some are :…; Gotchas: 2 a.m. can occur twice, or not at all, Sorting local times across zones is w…. An offset like +01:00 is not a time zone. Europe/London is — because only the name knows when the rules change.
Who is the Time & Time Zones poster for?
Time & Time Zones belongs to the Computing section rather than to a school year, because computing is not something one grade owns. Anyone learning foundations can pin it up — a beginner, a student mid-course, or someone revising years later.
When should you use the Time & Time Zones poster?
Store UTC. Convert only at the edges, for display. Never store a local time without its zone. A wall chart earns its place by being glanceable from where the work is happening, so Time & Time Zones belongs on the wall where that computing work actually happens, within glancing distance, rather than filed away.
What other posters go with Time & Time Zones?
Big-O Complexity, Binary, Hex & Decimal and Bits, Bytes & Sizes sit alongside Time & Time Zones in the Computing section. Printed together they make a wall rather than a single sheet, which is how a reference set actually gets used.Big-O ComplexityBinary, Hex & DecimalBits, Bytes & Sizes
Is the Time & Time Zones poster free to download and print?
Yes. Time & Time Zones downloads as a free PDF with no account, no email and no watermark, like everything else in the Computing section. Print as many copies as you like for a home, a classroom, a library or a tutoring group; reselling the file is the only thing the licence rules out.Read the licence
What size does the Time & Time Zones poster print at?
Time & Time Zones is a vector PDF laid out for US Letter, and prints on A4 with Fit to page — the same file, no separate download. Because every mark on it is drawn rather than photographed, it stays sharp enlarged to A3, A2 or A1 at a copy shop. Colour carries emphasis only, so a greyscale print of Time & Time Zones loses nothing.Printing guide

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