Using Tasmota in a completely offline configuration is extra special challenge. The use case was regularly operated events with no need for specific times. These are notes on how to accomplish that.

Time based triggers do not fire until Tasmota thinks it has a valid clock. A valid clock is defined as syncing up to an NTP server, or having a clock set by the Time command to something post 2015 or so.

This includes time based triggers in the Rule engine.

Thus, to get time based triggering to work, you need to run the Time command
after Tasmota has started.  See the first line, I'm working in a no network
environment.

So, Second thing I learned, the System#Boot trigger doesn't run where it's
name implies.  That trigger runs only after tasmota has successfully connected to the MQTT broker as a client, which is never going to happen in a no network environment.

The Power1#Boot trigger does happen very early however, thus you can use it to do Time setup, assuming your board has a Power1 channel/relay.

Rule1 ON Power1#Boot Do Time 1550000000 ENDON
Rule1 1
Rule2 On Time#Minute|30 DO Power1 On ENDON
Rule2 1

Field testing of a single module (n=1 wheee) indicated a no network attached
clock drift of about 3 minutes a day.  These are not the stable clocks you
are looking for.