I've read that RediSearch has support for dates and times via the "numeric" type, but there's not much information about this. Does this mean I just choose my own conversion from time, such as seconds since 1970?
Yep, exactly that (a.k.a epoch and timestamp).
You can also define your own epoch and timestamp resolution if you want, like "days since January 1 2018", as long as you convert to and from real times.
Also, the aggregation API has a bunch of functions augmenting dealing with timestamps. See https://oss.redislabs.com/redisearch/Aggregations/#list%22_%22of%22_%22datetime%22_%22apply%22_%22functions