<time> tag is not meant to be used for time periods such as decades or centuries. You should not use it to mark up
The documentation for the tag is here. It says that it can represent:
- a date
- a time
- a date and time
The machine readable representation of those dates and/or times is based on rfc3339 which states in the introduction:
Date and time expressions indicate an instant in time. Description of time periods, or intervals, is not covered here.
You could use two tags to cover the start and end of the period. For Example you could mark up
<time datetime="1961">1961</time>-<time datetime="1970">1970</time>. However when you use a phrase that covers the time period without mentioning its start and end times, it would not be appropriate to use one or more time tags.