You can make use of "year" function (Year("12/29/2010") will return 2010), on this basis u can select for the same year and get the sum.
If you want to pull up a client record and see the year to date total donations, do this:
Define an unstored calculation field in Client, cThisYear, as Year ( Get ( CurrentDate ) )
As Raj kumar suggests, define calculation field, cYear, in Donations as Year ( TransactionDateField ) // use your field name here
Define a new relationship:
Client::ClientID = DonationsThisYear::ClientID AND
Client::cThisYear = DontaionsThisYear::cYear
DonationsThisYear is a new table occurrence of your donations table. Select your original donations table occurrence in Manage | Database | Relationships and click the button with two plus signs to create it. Then link it as shown above to your existing Client table occurrence.
Now Sum ( DonationsThisYear::DonationAmt ) will compute the year to date total.