I needed a quick date format string today so thought I'd list two quick references.
From an MSDN page showing you how to generate all possible time patterns.
[0]: MM/dd/yyyy
[1]: dddd, dd MMMM yyyy
[2]: dddd, dd MMMM yyyy HH:mm
[3]: dddd, dd MMMM yyyy hh:mm tt
[4]: dddd, dd MMMM yyyy H:mm
[5]: dddd, dd MMMM yyyy h:mm tt
[6]: dddd, dd MMMM yyyy HH:mm:ss
[7]: MM/dd/yyyy HH:mm
[8]: MM/dd/yyyy hh:mm tt
[9]: MM/dd/yyyy H:mm
[10]: MM/dd/yyyy h:mm tt
[11]: MM/dd/yyyy HH:mm:ss
[12]: MMMM dd
[13]: MMMM dd
[14]: ddd, dd MMM yyyy HH':'mm':'ss 'GMT'
[15]: ddd, dd MMM yyyy HH':'mm':'ss 'GMT'
[16]: yyyy'-'MM'-'dd'T'HH':'mm':'ss
[17]: HH:mm
[18]: hh:mm tt
[19]: H:mm
[20]: h:mm tt
[21]: HH:mm:ss
[22]: yyyy'-'MM'-'dd HH':'mm':'ss'Z'
[23]: dddd, dd MMMM yyyy HH:mm:ss
[24]: yyyy MMMM
[25]: yyyy MMMM
From a google groups post by Anders Noras in microsoft.public.dotnet.general.
d :08/17/2000
D :Thursday, August 17, 2000
f :Thursday, August 17, 2000 16:32
F :Thursday, August 17, 2000 16:32:32
g :08/17/2000 16:32
G :08/17/2000 16:32:32
m :August 17
r :Thu, 17 Aug 2000 23:32:32 GMT
s :2000-08-17T16:32:32
t :16:32
T :16:32:32
u :2000-08-17 23:32:32Z
U :Thursday, August 17, 2000 23:32:32
y :August, 2000
dddd, MMMM dd yyyy :Thursday, August 17 2000
ddd, MMM d "'"yy :Thu, Aug 17 '00
dddd, MMMM dd :Thursday, August 17
M/yy :8/00
dd-MM-yy :17-08-00