I'm not familiar with ecowitt-client, so I'm not sure what code we are talking about.
In general, the type "ValueHelper" is used to format data. For data that are elapsed time, a "time context" is specified. The default is "current", which gives just minutes and seconds, as you witnessed. Perhaps the author of ecowitt-client forgot to specify a more appropriate context? For example, the "weewx uptime" and "system uptime" tags uses a time context of "month", which does what you want.
In any case, you can override this and specify the exact format you want. For example, if "$ecowitt_runtime" is the tag holding the runtime, then this should work (NOT TESTED):
$ecowitt_runtime.long_form("%(day)d%(day_label)s, %(hour)d%(hour_label)s, %(minute)d%(minute_label)s")
I didn't understand the comment, "This is written by the interceptor driver into the map, i.e. into the database 'runtime': 430624.0." What map? And how is the database entering this?
-tk