Say I have a question about an ancestor who served in the Second World War. Do I tag 20th-century, 1930s, 1940s, world-war-2, all of the above?

It will become unsustainable before long. We'll have some questions tagged with decades, some with centuries, some with neither. And that's not helpful for anyone looking for questions about those specific periods.

Here I propose removing the decade tags entirely. My reasoning:

• People don't know when to tag decades, and when to tag centuries. I don't know sometimes, even though I wrote the usage guidance for many of them. We subsequently have a mishmash of some questions tagged decades and some centuries.
• Some questions end up having 2 or more decade tags, which in some cases consumes valuable tagging real estate. Or the ones that should be tagged 2 decades only get one because there are more important tags.
• Any tag that has a decade tag could equally validly have a century tag. For example, with the current usage guidances there is no reason a 1920s question shouldn't also have the 20th-century tag – other than that would be plain silly.
• Decade tags usually serve minimal useful purpose in grouping questions. Questions rarely are specific to one decade. Near the top of the question list is: Meaning of “dofhoadayne” in 1648 Will of William Noake from Longburton, Dorset, England?. This question is not specific to the 1640s, but it is tagged as such. Palaeographically if makes little difference whether the question pertains to the 1640s or 1670s; hence the usefulness of a century tag which encompasses this entire period.
• To search for all questions in a multi-decade period, one has to do complex searching techniques (which I suspect the vast majority of users do not know how to do). Tags should cater to the moderately experienced user, rather than those who know the intricacies of the StackExchange search algorithms.

I support the tagging of events or conflicts in addition to the century, because the conflict tag conveys much more than just the time period.

This being said, in an ideal world I think it would be useful to be able to tag certain large record sets that are specific to a year or time period. For example, I think it could be nice to have a tag for uk-1881-census. Not for questions that mention the 1881 census, but for questions about the 1881 census. However, at this time I do not support such a proposal because I think it would become a worse tagging mess than the decades. The search function is adequate for findings questions that mention the 1881 census, even though it is not good for weeding out questions about the 1881 census.

The bottom line is that our tagging of dates needs to be simple and straightforward. Century tags are the answer to that. Adding any further tags such as decades or eras just makes tagging more confusing to the average user, which makes the tag groupings less useful to everyone.

Say I have a question about an ancestor who served in the Second World War. Do I tag 20th-century, 1930s, 1940s, world-war-2, all of the above?

It will become unsustainable before long. We'll have some questions tagged with decades, some with centuries, some with neither. And that's not helpful for anyone looking for questions about those specific periods.

Here I propose removing the decade tags entirely. My reasoning:

• People don't know when to tag decades, and when to tag centuries. I don't know sometimes, even though I wrote the usage guidance for many of them. We subsequently have a mishmash of some questions tagged decades and some centuries.
• Some questions end up having 2 or more decade tags, which in some cases consumes valuable tagging real estate. Or the ones that should be tagged 2 decades only get one because there are more important tags.
• Any tag that has a decade tag could equally validly have a century tag. For example, with the current usage guidances there is no reason a 1920s question shouldn't also have the 20th-century tag – other than that would be plain silly.
• Decade tags usually serve minimal useful purpose in grouping questions. Questions rarely are specific to one decade. Near the top of the question list is: Meaning of “dofhoadayne” in 1648 Will of William Noake from Longburton, Dorset, England?. This question is not specific to the 1640s, but it is tagged as such. Palaeographically if makes little difference whether the question pertains to the 1640s or 1670s; hence the usefulness of a century tag which encompasses this entire period.
• To search for all questions in a multi-decade period, one has to do complex searching techniques (which I suspect the vast majority of users do not know how to do). Tags should cater to the moderately experienced user, rather than those who know the intricacies of the StackExchange search algorithms.

I support the tagging of events or conflicts in addition to the century, because the conflict tag conveys much more than just the time period.

This being said, in an ideal world I think it would be useful to be able to tag certain large record sets that are specific to a year or time period. For example, I think it could be nice to have a tag for uk-1881-census. Not for questions that mention the 1881 census, but for questions about the 1881 census. However, at this time I do not support such a proposal because I think it would become a worse tagging mess than the decades. The search function is adequate for findings questions that mention the 1881 census, even though it is not good for weeding out questions about the 1881 census.

The bottom line is that our tagging of dates needs to be simple and straightforward. Century tags are the answer to that. Adding any further tags such as decades or eras just makes tagging more confusing to the average user, which makes the tag groupings less useful to everyone.