Tumblr imported blog showing wrong date

Option 1 (no code): you can manually edit each post and change the published date. Look in the “Publish” box where you publish or update posts, and there will be a “published on” date with an “edit” link.

Option 2 (requires database access): you can run a MySQL query to adjust all posts’ dates.

UPDATE wp_posts SET post_date = DATE_ADD(post_date,INTERVAL 1 DAY) WHERE post_type="post";
UPDATE wp_posts SET post_date_gmt = DATE_ADD(post_date_gmt,INTERVAL 1 DAY) WHERE post_type="post";

There are two lines because WP stores both a local timezone timestamp and a GMT timestamp.

This assumes your database uses the default wp_ prefix – you should first back up your database, then check which prefix you use, and if it’s a different prefix then just update wp_posts in both lines to whatever yours is called.