Dynamic averaging of a selectable date range

[zzzjtwzzz]

How do I setup a formula to calculate the average of a set of investments based on a selectable (Start Date and End Date) date range?

	Invest1	Invest2	Invest3	Invest4	Invest5		Start Date	End Date
Mth1	1.40	1.57	1.57	1.62	1.55		Mth3	Mth6
Mth2	1.29	1.72	1.71	1.73	1.71		Invest1	Avg Mth3-Mth6
Mth3	-1.86	-1.94	-1.94	-1.94	-1.93		Invest2	Avg Mth3-Mth6
Mth4	0.87	0.91	0.85	0.89	0.88		Invest3	Avg Mth3-Mth6
Mth5	0.68	0.72	0.72	0.70	0.76		Invest4	Avg Mth3-Mth6
Mth6	3.86	4.11	4.17	4.18	4.15		Invest5	Avg Mth3-Mth6
Mth7	-0.42	-0.61	-0.61	-0.66	-0.65
Mth8	0.93	1.01	1.02	1.02	1.05
Mth9	1.33	1.43	1.36	1.39	1.38
Mth10	6.38	6.75	6.81	6.84	6.78
Mth11	-0.74	-0.79	-0.79	-0.84	-0.83
Mth12	-5.30	-5.96	-5.95	-5.92	-5.94

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[Scott T]

	A	B	C	D	E	F	G	H	I
1		Invest1	Invest2	Invest3	Invest4	Invest5		Start Date	End Date
2	Mth1	1.4	1.57	1.57	1.62	1.55		Mth3	Mth6
3	Mth2	1.29	1.72	1.71	1.73	1.71		Invest1	0.8875
4	Mth3	-1.86	-1.94	-1.94	-1.94	-1.93		Invest2	0.95
5	Mth4	.87	.91	.85	.89	.88		Invest3	0.95
6	Mth5	.68	0.72	0.72	0.7	0.76		Invest4	0.9575
7	Mth6	3.86	4.11	4.17	4.18	4.15		Invest5	0.965
8	Mth7	-0.42	-0.61	-0.61	-0.66	-.65
9	Mth8	0.93	1.01	1.02	1.02	1.05
10	Mth9	1.33	1.43	1.36	1.39	1.38
11	Mth10	6.38	6.75	6.81	6.84	6.78
12	Mth11	-0.74	-0.79	-0.79	-0.84	-0.83
13	Mth12	-5.3	-5.96	-5.95	-5.92	-5.94

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In I3 and copy down

=AVERAGE(INDEX($B$2:$F$13,MATCH($H$2,$A$2:$A$13,0),MATCH($H3,$B$1:$F$1,0)):INDEX($B$2:$F$13,MATCH($I$2,$A$2:$A$13,0),MATCH($H3,$B$1:$F$1,0)))