When the 2014 National Curriculum was introduced, some degree of flexibility between year groups was encouraged. Y1/2 are linked and so are Y3/4 and Y5/6. So if a child had not understood a particular concept or skill introduced in, for example, Y5, the topic could be left until Y6 when it could be taught again with the child assessed at that point for understanding.
Similarly, a teacher can decide to leave a particular topic until the following year if they feel it is not appropriate for their particular class or group of children.
When I wrote the scope and sequence and developed lines of progression I decided that 24 hour time is better left until Y4. The focus in Y3 can then be on reading 12 hour clocks accurately and confidently in order to solve time problems - more than enough for children to get to grips with. Of course, higher attaining Y3 children can go on to explore 24 clocks, but it seems unnecessary content to add to the planning of an already full curriculum.
When I wrote the scope and sequence and developed lines of progression I decided that 24 hour time is better left until Y4. The focus in Y3 can then be on reading 12 hour clocks accurately and confidently in order to solve time problems - more than enough for children to get to grips with. Of course, higher attaining Y3 children can go on to explore 24 clocks, but it seems unnecessary content to add to the planning of an already full curriculum.