As physicskate said, your time on the Tier 1 PSW does not (cannot) count towards the 5 years to ILR, so even if you hadn't switched to FLR(M) but had switched to a different visa, it still wouldn't have counted.
As you will be applying for ILR as the spouse of a UK citizen, you have to spend 5 years in the UK in that category of visa in order to qualify for it... so your 5 year clock to ILR started the day your FLR(M) visa was granted - 21/12/15.
You will be eligible to apply for ILR anytime between 28 days before you reach 5 years on FLR(M) (28 days before 21/12/17) and the expiry date of your current visa - 17/12/17. So, you just need to make sure you apply between 23/11/17 and 17/12/17 (if I've counted back 28 days from 21/12/17 correctly).
If you had switched to FLR(M) before the rules changed in July 2012, you would have qualified for ILR in 2015, but as you are on the new post-July 2012 rules, you have to wait 5 years from the start of your first FLR(M) to qualify.