I did not have geoclue-ubuntu-geoip installed and installing it (+ logging out and back in) indeed fixed the problem, indicator-datetime-service is back to a reasonable memory consumption which does not seem to grow over time. indicator-datetime-service's dependence on "geoclue-ubuntu-geoip | geoclue-provider" was apparently fulfilled by geoclue-examples... judging from its name and description, this package should possibly not claim "Provides: geoclue-provider".
I did not have geoclue- ubuntu- geoip installed and installing it (+ logging out and back in) indeed fixed the problem, indicator- datetime- service is back to a reasonable memory consumption which does not seem to grow over time. indicator- datetime- service' s dependence on "geoclue- ubuntu- geoip | geoclue-provider" was apparently fulfilled by geoclue-examples... judging from its name and description, this package should possibly not claim "Provides: geoclue-provider".
Many thanks for the quick help!