I'm trying to make the timeline for Ted and Tracy's engagement through Penny's birth fit.
Here's the problem -
We know that Penny's at the "Star Wars" trilogy viewing in 2015. Whether she's a newborn or infant I'm not too concerned about, there are major labor issues with using newborn babies so Hollywood often uses infants instead.
All we know about when the Star Wars trlogy viewing occurs is it was when Ted and Marshall had finals. That would presumably mean May or December at most colleges.
In "The Lighthouse" it's stated twice that Ted wishes he was there with Tracy two years ago (instead of Cassie). That places Ted's first proposal to Tracy in 2015, presumably on the anniversary of the weekend Barney and Robin got married (last weekend of May 2013), although that isn't explicitly stated.
In "Last Forever, part one" Tracy says they can't get married in September because she won't fit into her wedding dress. Presumably she'll be showing by the second or third trimester. Obviously this scene has to take place after the proposal at the lighthouse and immediately after Tracy discovers that she's pregnant.
The problem is how can Tracy get pregnant after the proposal and still have Penny born in 2015?
The only solution I've been able to come up with assumes that Tracy's got irregular periods (so she doesn't immediately suspect she's pregnant when she misses one period), Penny was born premature, and the proposal took place earlier than assumed.
Here's the timeline -
Marchish 2015 - Ted and Tracy go to the Lighthouse to celebrate two years together (even though the anniversary of them getting together isn't until May). It's possible Penny's conceived on this trip. Afterwards they make preliminary plans to get married in September and start contacting their castle guy, hot air balloon guy, etc.
May 2015 - Tracy discovers that she's pregnant and tells Ted they can't get married in September because she won't fit into her wedding dress. At this point they may assume that the child is due in the January timeframe.
Novemberish 2015 - Penny's born premature.
December 2015 - Star Wars trilogy viewing.
Certainly the exact dates and be fiddled with - the trip to the lighthous and propsal could have been earlier (not likely it was later), Penny could have been born in early Decemeber with the trilogy viewing in late December, etc.
The alternate timeline which I had previously worked out had the proposal in May 2014, Tracy determines she's pregnant in June, and Penny born in early 2015, but that assumes that both "The Lighthouse" and "Last Forever Part 1" were incorrectly dated in 2015 when they should have been 2014. That timeline has the added benefit of having the triology viewing on one of the classic "Star Wars" holidays (May 4th - for the pun "May the Fourth be with you", or May 25th the anniversary of the day the first movie premiered.)