Standard Weeks are widely used in biological recording and these differ slightly from conventional or fiscal weeks.

The idea is that each calendar day is always in the same week whatever the year.

Standard weeks are defined as follows:

• Jan 1st always starts week 1
• If the year is a leap year then leap day (29th Feb) is included in week 9 and all subsequent days stay in the same week they would have been in if it were a non-leap year
• The last day of December, that would form a 'week 53', is included in week 52

For example, today (Fri 12th March 2010) is in Week 11.

A perpetual week calendar is given below (valid for any year):

Week	Dates (inclusive)
1	1st January to 7th January
2	8th January to 14th January
3	15th January to 21st January
4	22nd January to 28th January
5	29th January to 4th February
6	5th February to 11th February
7	12th February to 18th February
8	19th February to 25th February
9	26th February to 4th March (including 29 February in a Leap Year)
10	5th March to 11th March
11	12th March to 18th March
12	19th March to 25th March
13	26th March to 1st April
14	2nd April to 8th April
15	9th April to 15th April
16	16th April to 22nd April
17	23th April to 29th April
18	30th April to 6th May
19	7th May to 13th May
20	14th May to 20th May
21	21st May to 27th May
22	28th May to 3rd June
23	4th June to 10th June
24	11th June to 17th June
25	18th June to 24th June
26	25th June to 1st July
27	2nd July to 8th July
28	9th July to 15th July
29	16th July to 22nd July
30	23rd July to 29th July
31	30th July to 5th August
32	6th August to 12th August
33	13th August to 19th August
34	20th August to 26th August
35	27th August to 2nd September
36	3rd September to 9th September
37	10th September to 16th September
38	17th September to 23rd September
39	24th September to 30th September
40	1st October to 7th October
41	8th October to 14th October
42	15th October to 21st October
43	22nd October to 28th October
44	29th October to 4th November
45	5th November to 11th November
46	12th November to 18th November
47	19th November to 25th November
48	26th November to 2nd December
49	3rd December to 9th December
50	10th December to 16th December
51	17th December to 23rd December
52	24th December to 31st December (note this is an 8 day week)