Time functions in zabbix triggers

[lulu]

Hi,

I need to write an alert that triggers only once a day if data is missing

{myhost:myitem.time(0)}>{$MACRO_START_TIME} and 
{myhost:myitem.time(0)} <{$MACRO_START_TIME} + 10MIN
and
{myhost:myitem.nodata(10m)}

How can I calculate

{$MACRO_START_TIME} + 10MIN

Or is there a better way to write that?

thanks in advanvce