I was getting the same warning as the following rt-devel post which I found online - though I’m not subscribed to so I’m posting my fix here instead.
[warning]: Warning: Use of "localtime"
without parentheses is ambiguous at
/opt/rt4/sbin/../lib/RT/Interface/Email.pm line 526.
Correction is to edit Email.pm and where it has “localtime”, use “localtime()” instead. Cursory search online for perl localtime shows examples with correct syntax.
Note: Am working in RT 4.2.11