Max function with time constraints

try this array formula:

=MAX(IF($A$2:$A$794=G2,IF($C$2:$C$794=H2,$D$2:$D$794)))-MIN(IF($A$2:$A$794=G2,IF($C$2:$C$794=H2,$D$2:$D$794)))

Array Formulae are a little different from ordinary formulae in that they MUST be confirmed by pressing CTRL+SHIFT+ENTER to activate the array, not just ENTER.

You will know the array is active when you see curly brackets { } - or "curly braces" for those of you in the USA, or "flower brackets" for those of you in India - appear around the outside of your formula. If you do not use CTRL+SHIFT+ENTER you will (almost always) get an error message or an incorrect answer. Press F2 on that cell and try again.

Don't type the curly brackets yourself - it won't work...

So the time period of collection is irregular each day? the process is not being monitored 24/7??

No reply yet. The easiest way is to assume that the process is 24/7 and use this to set the date (G2, copied down):
=$A$2+INT((ROWS($G$2:G2)-1)/24)

this to set the time(H2, copied down):
=MOD((ROWS($H$2:H2)-1),24)

and this array formula to return your value:
=IF(MAX(IF($A$2:$A$794=G2,IF($C$2:$C$794=H2,$D$2:$D$794)))=0,"",MAX(IF($A$2:$A$794=G2,IF($C$2:$C$794=H2,$D$2:$D$794)))-MIN(IF($A$2:$A$794=G2,IF($C$2:$C$794=H2,$D$2:$D$794))))

If this is not what you want , then please specify EXACTLY what you want, rather than leave us guessing.