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Priya AakankshaPriya Aakanksha 

Field is not writeable: Opportunity.IsWon

error is showing in batch class
1)Method does not exist or incorrect signature: void day() from the type User
2)Field is not writeable: Opportunity.IsWon 
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if(Opp.LastModifiedBy.day() > system.today()+30){
                opp.LofB__c='Retail';
              opp.ISWON ='false';
Priya AakankshaPriya Aakanksha
lastmodifiedby is lookup to userand its a standard field
ISWON is hidden field i got that field name after extract the data.