In a TAF, for how long is the weather forecast valid if it states "valid from 22nd day at 1700Z to 23rd day 0500Z"?

The correct answer is that the weather forecast is valid for 24 hours. This is determined by the start and end times specified in the TAF (Terminal Aerodrome Forecast). In the given case, the forecast begins on the 22nd day at 1700Z and ends on the 23rd day at 0500Z.

To calculate the duration, you take the ending time of 0500Z on the 23rd and subtract the starting time of 1700Z on the 22nd. This results in a time span of 12 hours on the 22nd (from 1700Z to midnight) plus 5 hours on the 23rd (from midnight to 0500Z), which totals to 17 hours. However, it should be noted that the TAF encompasses the complete duration until 0500Z on the 23rd, giving full validity of the forecast until the end of that hour.

Therefore, since the forecast starts at 1700Z and ends at 0500Z the next day without any gaps, the entire period constitutes a 24-hour duration. This ensures that any necessary adjustments or anticipations for flight operations can be made according to the comprehensive weather forecast provided