These numbers, 1218/1324, 1220/1222, 1306/1308, express a period of time. 

The first two digits indicate the DATE and the last two digits indicate the hour in Zulu time.

Here’s an example:

KMEM 121720Z 1218/1324 20012KT 5SM

121720Z — Report was issued on the 12th of the month at 1720Z

1218/1324 — starting on the 12th at 1800Z until the 13th at 2400Z

20012KT — winds are from 200º at 12 knots

Additionally, you can use the TAF Decoder tool available on the National Weather Service website to help you better understand the breakdown of TAF reports.