I've had a quick test and this should do the trick:

RewriteEngine on
RewriteRule ^([0-9]+)/([0-9]+)$ /date/$1/$2 [R=301,L]

I've had a quick test and this should do the trick:

RewriteEngine on
RewriteRule ^([0-9]+)/([0-9]+)$ /date/$1/$2 [R=301,L]

You may also need to exclude this from the redirect to the index.php page by adding

RewriteCond %{REQUEST_URI} !^/(date) [NC]

so it should look like this

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^/(date) [NC]
RewriteRule . /index.php [L]

RewriteRule ^([0-9]+)/([0-9]+)$ /date/$1/$2 [R=301,L]
</IfModule>
# END WordPress

I haven't tried it but you could try:

RewriteEngine on
RewriteRule ^([0-9]+)/([0-9]+)$ /date/$1 [R=301,L]

I've had a quick test and this should do the trick:

RewriteEngine on
RewriteRule ^([0-9]+)/([0-9]+)$ /date/$1/$2 [R=301,L]