In a database, is it better to store a time period as a start/end date, or a start date and a length of time?

Question

This is a completely hypothetical question: let's say I have a database where I need to store memberships for a user, which can last for a specific amount of time (1 month, 3 months, 6 months, 1 year, etc).

Is it better to have a table Memberships that has fields (each date being stored as a unix timestamp):

user_id INT, start_date INT, end_date INT

or to store it as:

user_id INT, start_date INT, length INT

Either way, you can query for users with an active membership (for example). For the latter situation, arithmetic would need to be performed every time that query is ran, while the former situation only requires the end date to be calculated once (on insertion). From this point of view, it seems like the former design is better - but are there any drawbacks to it? Are there any common problems that can be avoided by storing the length instead, that cannot be avoided by storing the date?

Also, are unix timestamps the way to go when storing time/date data, or is something like DATETIME preferred? I have run into problems with both datatypes (excessive conversions) but usually settle on unix timestamps. If something like DATETIME is preferred, how does this change the answer to my previous design question?

Answer 1

It really depends on what type of queries you'll be running against your date. If queries involve search by start/end time or range of dates then start/and date then definitely go with first option.

If you more interested in statistic (What is average membership period? How many people are members for more than one year?) then I'd chose 2nd option.

Regarding excessive conversion - on which language are you programming? Java/Ruby use Joda Time under hood and it simplifies date/time related logic a lot.