Spousal Death property value

My Mother lost her husband in Nov. of 99.
This is a second marriage for both of them. They decided that whomever should pass away first, would divide the estate equally with the remaining children. This has been set up in a living trust. My question is this, When is the appraisal of the property ( I refer specifically to the house itself.) valid? Is it at the time of death, or the time on which she may choose to buyout the remaining parties?(in the living trust they stipulated that the remaning spouse had up to two years to either sell or buyout.) I have heard conflicting opinions on this issue, and would like to put her mind at ease, as her property easily appreciates by at least $1,000.00 a month as she resides in California.

It depends on their specific agreement and the exact terms of the trust(s).

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Thank you for your reply.
There were no specific terms that addressed when the appraisal would be valid. The trust stated that whomever should pass on first would recive $1,000.00 a month for two years. If either before or after the two year limit had expired they then could either sell the home and split the proceeds with the deceased relatives or buyout there half. According to Calif law, is the time of death when the appraisal is considered valid, or at the time of sale?