Don't ask for more precision then you actually need (Don't ask date & time when you only need year & week)
Don't store dates and times with more precision then was actually entered (e.g. Don't ask for year & week, and then store the (calculated) first day of the week.
On my first large project I made the mistake of asking for the year & week while storing the calculated first day of the week, however using the wrong first day of the week and the wrong numbering of weeks…
The first day of the week can be sunday or monday depending on where you are.
The way to count the weeks isn't the same everywhere.
Some advice:
Default to using ISO 8601 whenever possible http://en.wikipedia.org/wiki/ISO_8601
Don't ask for more precision then you actually need (Don't ask date & time when you only need year & week)
Don't store dates and times with more precision then was actually entered (e.g. Don't ask for year & week, and then store the (calculated) first day of the week.
On my first large project I made the mistake of asking for the year & week while storing the calculated first day of the week, however using the wrong first day of the week and the wrong numbering of weeks…