in Met my Sister on a Dating Site Ayumu gives a specific date for when the story starts: New Year's Eve 2016. It's a least a few days later when she meets up with Sayo, making it now Jan 2017 (they later say it's been 333 days since this meeting when they get married on Christmas Eve 2017, meaning they met Jan 25, more than a few days after New Years). Sayo is 21 at that time, and since they have a 5 year age gap this makes Ayumu 25-27 depending on when their birthdays are, though 26 is by far the most likely.
Ayumu also appears in Yuri Natsu, while she's in university. She failed her entrance exams and took a gap year, so she started uni at 19, and judging from the amount of free time she has to not be job hunting, she's probably not in her fourth year yet. This gives us an age range of 19-22. Which means Yuri Natsu must take place within the range of 2008-2013. (The 2008 or 2013 dates are both extremely unlikely though. Ayumu is almost certainly 26 in jan 2017, so Yuri Natsu is much more likely in the 2009-2012 range.)
In Yuri Natsu, Hana is in an unspecified year of middle school, while in Wagayuri she's in grade 10. Which means Wagayuri takes place at least 1 but not more than 3 years after Yuri Natsu, giving us a date range of 2009-2016.
(Considering the aforementioned unlikelihood of the most extreme dates, that gives us a "more likely" date range of 2010-2015)
For a couple reasons involving things Miki says in Yuri Natsu and later in this manga, I suspect it's only been 1-2 years since the events of Yuri Natsu, meaning 2015 is actually the most recent year this manga could take place, with a "more likely" date range of 2010-2014. But I can't confirm that yet, so it might be still be 3 years.