The times on a TAF are hours only, because it is a
prediction onto the future. No Met man (unless he has heavenly powers) can "see"
that accurately into the future. The temp and QNH forecast therefore covers a block where the start time is the 'earliest' this phenominon will happen (before the published hour) and the end time is the 'latest' it is predicted to happen (slightly after the published time)
Remember: The future is the worlds best kept secret.
A METAR, on the other hand, is recent 'history'. It has happened already. It was physically measured during the
10 min prior to its publication time, on the hour e.g. 0800 UTC and, at International airports also on the 1/2 hour e.g. 0800, 0830, 0900, 0930 etc
A SPECI will give you the exact time of observation, hours and minutes, because it is 'history' also, measured physically or observed.
Read the AIP Chapter 3.5 MET section. You will now understand it better..
