Sum column values depending of date range

[girapas]

I have a dynamic database, filled everyday with data in some columns. I enter, in column C a Date and in column G a number. Other columns contain strings.
What I want is to get the SUM of dynamically filled column G numbers, for these cases:
1) From the beginning of the current year (January, 1) to today.
2) For each quarter and half-year.

Thanks in advance