Timelag in indicators has up to now not been supported in DHIS2 (it was supported in DHIS 1.x). I recall that an issue was raised some 2-3 years ago to add this, but I cannot recall the issue name and I’m not sure if the issue was transferred to JIRA when JIRA was introduced in late 2016.
It was raised again in JIRA issue DHIS2-3142 just over one year ago. That issue seems to cover more or less what you are looking for. I am not sure why it hasn’t moved forward, though - conceptually, offsetting the period used in a denominator seems rather straightforward - but it could be the current design of the analytics data model and the “indicator engine” are such that introducing offsets would require massive changes. I’m not sure…