AddYear not recognised?

Hi again,

Any idea why it says it cant find the addYear function when i add this script:

For each ServDue1 in Customer_Database [ServDue1 < zoho.currentdate]
{
ServDue1=ServDue1.addYear(1) ;
}

Yet if i put this

For each ServDue1 in Customer_Database [ServDue1 < zoho.currentdate]
{
ServDue1.addYear(1) ;
}

It says built in functions can only be in the format x=y.z()

So it is clearly recognising it in the second instance but when I call it correctly it can't find it??

Thanks

Sam