I was not happy with the output format of syslogd events since the time resolution was not enough and not sortable. I did set in my rc.conf:
$ sysrc -n syslogd_flags
-N -ss -O rfc5424

These periodics are then rendered useless:
$ grep -r "date " /etc/periodic
/etc/periodic/daily/460.status-mail-rejects:        yesterday=$(date -v-1d '+%b %e')
/etc/periodic/daily/460.status-mail-rejects:        today=$(date '+%b %e')
/etc/periodic/security/900.tcpwrap:yesterday=`date -v-1d "+%b %e "`
/etc/periodic/security/800.loginfail:yesterday=`date -v-1d "+%b %e "`

They rely on the old style formats and miss everything highres timestamps.

So a possible solution is to query "sysrc -n syslogd_flags" and use the appropriate format:
today=$(date -I)
yesterday=$(date -v-1d -I)
or old style

to filter out events.

Manpage of syslogd says:
     -O format
             Select the output format of generated log messages.  The values
             bsd and rfc3164 are used to generate RFC 3164 log messages.  The
             values syslog and rfc5424 are used to generate RFC 5424 log
             messages, having RFC 3339 timestamps with microsecond precision.
             The default is to generate RFC 3164 log messages.

So it is '-O bsd|rfc3164' or '-O syslog|rfc5424'. This could be done with grep or sed.