Is it better to shield using T-Bill if birthday start or ending or the month?
Maybe, maybe not. It depends on the T-bill rate you can get. When your 55th birthday falls near the “edge” of the calendar month (either edge) the cost of “shielding“ SA dollars using a bond unit trust might be 2 months of lost SA interest instead of 1. The cost of shielding using a 6 month T-bill is always 7 months of lost SA interest. (A few T-bills cost 8 months of lost SA interest. Don’t choose those!) So, is the T-bill rate high enough to earn the equivalent of 5 months of SA interest (the difference between 7 and 2)? That’s the basic question, and that’s just a math question that you need to compute.
Note that T-bills are issued in $1,000 face value increments, and they’re issued about every 2 weeks. So it’s difficult or impossible to sweep up all the SA dollars that can be shielded using T-bills alone. You will still probably want to use a bond unit trust to sweep up “stray” SA dollars even if you use T-bills for shielding and even if the unit trust-based method involves 2 months of SA interest loss for you.
Let’s try an example. Let’s suppose the SA interest rate is 4.08% p.a. (as now) and that you can buy a 6 month T-bill for $982. (That is, you pay $982 and get back $1,000 after 6 months — and with 7 months of lost SA interest.) Here’s the computation for 5 months of SA interest at 4.08% on $982:
$982 * (5/12) * 4.08% = $16.69
$16.69 is less than the $18 you get with the 6 month T-bill, so in this case the T-bill is the winner for someone with a birthday falling too close to the edge of the calendar month. (Although the bond unit trust can still be used to shield “stray” dollars.) Now let’s look at whether this same T-bill would be better than a bond unit trust for someone who only needs to lose 1 month of SA interest with bond unit trust-based shielding:
$982 * (6/12) * 4.08% = $20.03
And the answer is no. $20.03 (the lost SA interest minus 1 month of lost SA interest) is greater than $18 (the interest earned on the T-bill). And it’s not a tiny difference, so in this case you’re better off using the unit trust-based method even if the bond unit trust wobbles a bit in the few days you’re holding it.
It’s a little more complicated since the 4.08% SA rate in this example is above the floor rate and likely to change starting April 1, but that’s the basic idea.