I am attempting to scan for the current Weekly ATM contracts -1/+1 on SPY on each slice.

I have setup my "Options Filter Universe" like so (0-6 days)

var option = AddOption("SPY", Resolution.Minute);
option.SetFilter(u => u.WeeklysOnly()
.Strikes(-1, 1)
.Expiration(TimeSpan.Zero, TimeSpan.FromDays(6)));

So at any given point I should be returning the contract that expires on the Friday of the current trading week - correct?  

Next I want to extract the symbol of those two contracts on each data slice:

public override void OnData(Slice slice)
{
foreach (KeyValuePair<Symbol, OptionChain> chain in slice.OptionChains){
if (chain.Key != option_symbol) continue;
OptionChain optionsChain = chain.Value;
OptionContract[] contracts = optionsChain.ToArray();
if(contracts != null && contracts.Length > 0){
// find the day the contract expires
// (all options in chain should have same Expiry due to filters?)
this.last_trading_day = contracts[0].Expiry.Date;
}
}
}

And finally, I want to print that date as a sanity check:

Schedule.On(DateRules.EveryDay("SPY"), TimeRules.AfterMarketOpen("SPY", 0), () => {
if(this.last_trading_day != null){
Debug("Time: "+Time+" Next Expiry is: "+this.last_trading_day.ToShortDateString());
});

But Instead I am getting debug entires like (using 9/1 to 10/31/18), Fridays are: (9/7, 9/14, 9/21, 9/28):

Time: 9/4/2018 9:30:00 AM Next Expiry is: 1/1/0001
771 | 03:24:14:
Time: 9/5/2018 9:30:00 AM Next Expiry is: 9/5/2018
772 | 03:24:16:
Time: 9/6/2018 9:30:00 AM Next Expiry is: 9/5/2018
773 | 03:24:19:
Backtest deployed in 18.532 seconds
774 | 03:24:19:
Time: 9/7/2018 9:30:00 AM Next Expiry is: 9/7/2018
775 | 03:24:21:
Time: 9/10/2018 9:30:00 AM Next Expiry is: 9/10/2018
776 | 03:24:23:
Time: 9/11/2018 9:30:00 AM Next Expiry is: 9/10/2018
777 | 03:24:25:
Time: 9/12/2018 9:30:00 AM Next Expiry is: 9/12/2018
778 | 03:24:27:
Time: 9/13/2018 9:30:00 AM Next Expiry is: 9/12/2018
779 | 03:24:30:
Time: 9/14/2018 9:30:00 AM Next Expiry is: 9/14/2018
780 | 03:24:32:
Time: 9/17/2018 9:30:00 AM Next Expiry is: 9/17/2018
781 | 03:24:34:
Time: 9/18/2018 9:30:00 AM Next Expiry is: 9/17/2018
782 | 03:24:35:
Time: 9/19/2018 9:30:00 AM Next Expiry is: 9/19/2018
783 | 03:24:37:
Time: 9/20/2018 9:30:00 AM Next Expiry is: 9/19/2018
784 | 03:24:39:
Time: 9/21/2018 9:30:00 AM Next Expiry is: 9/19/2018
785 | 03:24:41:
Time: 9/24/2018 9:30:00 AM Next Expiry is: 9/19/2018
786 | 03:24:43:
Time: 9/25/2018 9:30:00 AM Next Expiry is: 9/19/2018
787 | 03:24:45:
Time: 9/26/2018 9:30:00 AM Next Expiry is: 9/19/2018
788 | 03:24:47:
Time: 9/27/2018 9:30:00 AM Next Expiry is: 9/19/2018
789 | 03:24:49:
Time: 9/28/2018 9:30:00 AM Next Expiry is: 9/19/2018
790 | 03:24:51:
Time: 10/1/2018 9:30:00 AM Next Expiry is: 9/19/2018
791 | 03:24:53:
Time: 10/2/2018 9:30:00 AM Next Expiry is: 9/19/2018
792 | 03:24:55:
Time: 10/3/2018 9:30:00 AM Next Expiry is: 9/19/2018