Quote from Kevin Schmit:
You can easily do it with just one extra intermediate cell. For
example, to grab the value of cell c2 in cell f2:
c2 is [dde emini link]
d2 is =if(MOD(now(0),1)<time(12,0,0),c2,d2)
e2 is =if(MOD(now(0),1)>=time(12,0,0),d2,0)
Cell e2 will display zero before noon, the noon emini value
at noon and later. Cell d2 stops changing at noon.
Not that cell d2 is self-referencial. For this to work you have
to go to tools/options/calculation and check the Iteration box.