Excel Excel get a calculated value to land on a specific date

[Corinneo]

This is a bit tricky to explain:

I have dates along the X axis and quantities up the Y axis.

When I enter a quantity on a date I need a formula to calculate
a) 3 times the entered quantity and then again 6 times the entered value (easy)
b) for these two values to automatically land on the 6th and 12th month from the date of entry (not so easy)

I need help with (b)

I'm picturing this as an ongoing line graph with new series being added periodically.

Is this possible or am I making this unnecessarily difficult for myself?

Thanks

Corinne