I've always found it inconvenient translating dates into timestamps and vice versa. The converters I've seen didn't seem very useable to me. So I made my own.
Tmstmp translates dates into timestamps, and time intervals into the number of milliseconds in those intervals. It has two fields. In the top one you write the date, in the bottom one you get the result. (And vice versa.)
I've tried to collect all the common date formats that I've been translating lately. You can insert or enter the date in the field, and then “it'll just work”.