[Remind-Fans] trigger date + x days

[Jorey Bump]

Paul Pelzl wrote:
> On Mon, May 14, 2007 at 02:39:57PM +0100, T E Schmitz wrote:
>> I would like to specify a trigger date as date+x days. What's the syntax 
>> for this?
>>
>> I am looking for something like
>>
>> REM [trigger('2007-04-11' +180] MSG top up phone-card
> 
> This works, if you add the closing ')' for the trigger() function...

How would you write it to trigger *every* 180 days from that date?