I don’t use the official client, so hopefully someone else can give you a detailed answer. I can just see several thing wrong with your cron there though.
looking at this thread on renewal you should be able to just do a “letsencrypt renew” if you have the latest version of the official letsencrypt client.
Looking at what you have passed you have used the argument “–renew-by-default” which will renew irrespective of how long the current certificate has left. You need to remove that parameter from your command.
The third variable of the cron ( *60 in your case ) - is the day of the month. Since there are less than 31 days in a month, the value you have there is invalid. Hence it’s running at midnight every day and getting a new cert.