How to: value(now) - value(some time ago) to see Difference over time?

You can do it per panel with the Graph panel timeshift override:

http://docs.grafana.org/reference/timerange/#panel-time-overrides-timeshift

But if you want to do per query then InfluxDB does not seem to currently support any timeshift functions:

https://github.com/influxdata/influxdb/issues/142 (and you can see here that lag variables have not been implemented yet)

There seems to be a temporary solution suggested here:
https://github.com/influxdata/influxdb/issues/142#issuecomment-291492226

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