[R] Regex in an IF statement?

[Jim Hargreaves]

Dear List

I have the POSIX timestamps of two events, A and B respectively. I want 
an expression that tests if A occurs within 4 weeks of B.

Something like:

if (ts_A "is between" ts_B + 4(weeks) and ts_B - 4) ...

Can probably done by regex but the regex docu for R says nothing about 
if statements.

Any help greatly appreciated,

Regards,
Jim Hargreaves