how should i handel datetime

Dear all

i have a two servers, one is for asp.net app and other is for sqlserve
1)how should i treat datetime in strored procedure and in c# code please note i do not want to take risk(some body may change the format of date in windows or other places (except sqlserver internally))
2) Please sugggest me for windows app which works on singel desktop where app and sqlserver both are on same machine and database can be on other machine also. yours sincerelly,
Note: If some one changes date format of windows OS/system formte will it affect the code sqlserver/ c#.
...