<P> Historically GMT has been used with two different conventions for numbering hours . The long - standing astronomical convention dating from the work of Ptolemy, was to refer to noon as zero hours (see Julian day). This contrasted with the civil convention of referring to midnight as zero hours dating from the Romans . The latter convention was adopted on and after 1 January 1925 for astronomical purposes, resulting in a discontinuity of 12 hours, or half a day . The instant that was designated' December 31.5 GMT' in 1924 almanacs became' January 1.0 GMT' in 1925 almanacs . The term Greenwich Mean Astronomical Time (GMAT) was introduced to unambiguously refer to the previous noon - based astronomical convention for GMT . The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours . </P> <P> Legally, the civil time used in the UK is called still "Greenwich mean time" (without capitalisation), according to the Interpretation Act 1978, with an exception made for those periods when the Summer Time Act 1972 orders an hour's shift for daylight saving . The Interpretation Act 1978, section 9, provides that whenever an expression of time occurs in an Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time . Under subsection 23 (3), the same rule applies to deeds and other instruments . </P> <P> During the experiment of 1968 - 1971, when the British Isles did not revert to Greenwich Mean Time during the winter, the all - year British Summer Time was called British Standard Time (BST). </P> <P> In the UK, UTC + 0 is disseminated to the general public in winter and UTC + 1 in summer . </P>

Greenwich mean time (gmt) is the standard time of which of the following countries