Sorry very basic libreoffice time calculations hours

[DLX]

I have looked at videos, bit don't get right out come  :-[ My maths just as great and my English.
using libreoffice spreadsheet

what would like is to add up hours from on time to end time (should be easy)

I put start time in column B Cell 2 = B2 and end time in column C Cell 2

10:00 B2 15:00 C2 in column D Cell 2 =C2-B2 then hit enter done show me hours from start to end.
once I get the hours I would like a total then up to.

I also clicked in cell format cell to numbers and get error Err:552[table][tr][td][/td][/tr][/table]Thanks
Sorry did say very basic.



- 15:15 why -
I know i'm doing this the slow way =Cx - Bx how would add left column B to C column please.
instead of

=C2-B2
=C3-B3
=C3-B4
=C5-B5
so on


thanks  :-[




Thanks