Civ. Code, § 1511

The want of performance of an obligation, or of an offer of performance, in whole or in part, or any delay therein, is excused by the following causes, to the extent to which they operate:

1. When such performance or offer is prevented or delayed by the act of the creditor, or by the operation of law, even though there may have been a stipulation that this shall not be an excuse; however, the parties may expressly require in a contract that the party relying on the provisions of this paragraph give written notice to the other party or parties, within a reasonable time after the occurrence of the event excusing performance, of an intention to claim an extension of time or of an intention to bring suit or of any other similar or related intent, provided the requirement of such notice is reasonable and just;

2. When it is prevented or delayed by an irresistible, superhuman cause, or by the act of public enemies of this state or of the United States, unless the parties have expressly agreed to the contrary; or,

3. When the debtor is induced not to make it, by any act of the creditor intended or naturally tending to have that effect, done at or before the time at which such performance or offer may be made, and not rescinded before that time.