Simple Network Management Protocol (SNMP) is used in network management systems to monitor network-attached devices for conditions that warrant administrative attention.
You can add an OID to snmpd.conf like this:
extend-sh unixtime date "+%s"
You can get the output like this:
snmpget -v2c -cpublic localhost NET-SNMP-EXTEND-MIB::nsExtendOutput1Line.\"unixtime\"
There's a default timeout of 5 seconds:
snmpget -v2c -cpublic localhost NET-SNMP-EXTEND-MIB::nsExtendCacheTime.\"unixtime\"
This timeout seems to be changeable with a set command, but unfortunately that doesn't seem to survive a restart of snmpd.