Enter this line of code in your subscription form (but choose whichever custom field makes sense to you):
<txp:bab_pm_zemTime custom_field="zemSubscriberCustom2" />
That should be it.
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Official Documentation for Postmaster (A Textpattern Plugin)
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Postmaster is a plugin for Textpattern, and is licensed under the GNU GPL. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public Licens for more details.