A TAF valid period is written 1212/1318. What does this mean?

A terminal aerodrome forecast's validity period is written as two groups separated by a slash, each combining a day of the month with a whole hour in UTC. In 1212/1318 the first group is the 12th day at 1200Z and the second is the 13th day at 1800Z, so the forecast covers 30 hours spanning a date change. Reading the digits correctly requires remembering that the first two digits of each group are a day, not an hour — the most common error is reading 1212 as twelve minutes past twelve. Everything in aviation weather is in UTC, which for a pilot in the continental United States means the forecast day can differ from the local day, particularly for evening and early morning operations. TAFs are issued for a specific airport and are valid within roughly five statute miles of it, so a distant site is informed by the nearest TAF rather than governed by it.

Each group combines a day and a whole hour in UTC, so 1212/1318 runs from the 12th at 1200Z to the 13th at 1800Z — a 30-hour period spanning a date change.

TAF validity periods are written as DDHH/DDHH in UTC, where the first two digits of each group are the day of the month. TAFs apply to a specific airport and roughly a five statute mile radius, and are typically issued four times daily.

Day-Hour, Day-Hour. The first two digits are always the date — 1212 is the 12th at noon Zulu.

The forecast applies within roughly five statute miles of the airport. For a site further out, the nearest TAF is guidance rather than a governing forecast.

• •Reading the leading digits as hours or minutes
• •Treating the times as local rather than UTC
• •Applying a TAF to a site well beyond the airport's vicinity