Compare DateTime Values

guys this is kind of a biggie for me.
i've only used sql server to compare the Date Portion of date(s), not the whole string (mm/dd/yyyy hh:mi).
i have a table called Time_Check which has two fields with timestamps,
"TimeLastRun & TimeNextRun". i was trying to compare the Current TimeStamp that i pass from a vb.net form to a stored procedure and compare that DateTime to the TimeNextRun. I'd like to then Update the two Time Columns if rows are found. Should i maybe be using the DateDif Func if i know ahead of time how much time should be elapsed(say five minutes)?
Any Code help is appreciated

Create Procedure dbo.My_Time_Check
@myDate smalldatetime
as
select alertnumber,alertname,TimeLastRun,TimeNextRun
from time_check
where timeNextRun >= @myDate
--would like to update these two columns if rows are found

thanks again
this forum is the best
rik