Am am running PR 1.2 (V 10.2010.19-1) but I can't find this feature.
I find some configuration files of modest

Account settings:

/var/lib/gconf/apps/modest/server_accounts/mymail@46@org@64@gmail@46@comID_transport/\%gconf.xml

<?xml version="1.0"?>
<gconf>
<entry name="security" mtime="1283436519" type="string">
<stringvalue>ssl</stringvalue>
</entry>
<entry name="auth_mech" mtime="1283436519" type="string">
<stringvalue>password</stringvalue>
</entry>
<entry name="port" mtime="1283436519" type="int" value="465"/>
<entry name="proto" mtime="1283436519" type="string">
<stringvalue>smtp</stringvalue>
</entry>
<entry name="password" mtime="1283436519" type="string">
<stringvalue>mypass</stringvalue>
</entry>
<entry name="username" mtime="1283436519" type="string">
<stringvalue>myaccount</stringvalue>
</entry>
<entry name="hostname" mtime="1283436519" type="string">
<stringvalue>smtp.gmail.com</stringvalue>
</entry>
</gconf>


and store

<?xml version="1.0"?>
<gconf>
<entry name="username_succeeded" mtime="1283436522" type="bool" value="true"/>
<entry name="security" mtime="1283436519" type="string">
<stringvalue>ssl</stringvalue>
</entry>
<entry name="auth_mech" mtime="1283436519" type="string">
<stringvalue>none</stringvalue>
</entry>
<entry name="port" mtime="1283436519" type="int" value="993"/>
<entry name="proto" mtime="1283436519" type="string">
<stringvalue>imap</stringvalue>
</entry>
<entry name="password" mtime="1283436519" type="string">
<stringvalue>mypass</stringvalue>
</entry>
<entry name="username" mtime="1283436519" type="string">
<stringvalue>mymail</stringvalue>
</entry>
<entry name="hostname" mtime="1283436519" type="string">
<stringvalue>imap.gmail.com</stringvalue>
</entry>
</gconf>

Nothing about Imap max. date...