node.js - if input maybe undefined how to use swig date filter -

i use swig in nodejs project，when use date filter，i found this：

{{t.updateat|date('y-m-d h:i:s',-480)}} nan-nan-nan nan:nan:nan 

because t.updateat undefined, when time need date filter output ''.

so, how can do?

{% if t.updateat %}     {{t.updateat|date('y-m-d h:i:s',-480)}} {% endif %}