It gets inserted as "000..." because you pass an invalid date/timsetamp value - don't use localtime in scalar context, but in list-context and use the resulting array to assemble a string as described in the MySQL documentation.
But as I tried to say: You don't need to format it, as you should be able to use FROM_UNIXTIME to convert a unix-timestamp (seconds since epoch) into something mysql is able to treat as an sql-timestamp.

What I don't understand is, what you want to achieve, as you don't get a date by subtracting old_date from now(), that could be used as a timestamp in a meaningful way, but an intervall in seconds. You shouldn't store these as timestamp-values but as integers - as they aren't timestamps (referring to a point in time).

In short: I guess you try to solve the wrong problem, or you didn't state the real problem - or maybe both..., nevertheless reading the docs I pointed you to should enable you to format the values you have to solve your stated problem in the way you want - namely inserting the difference as sql-timestamp.