"I have been a software engineer at [ name the company] for the past 18 months, and counting. I do this, this and that" It's all a matter of personal preference.
You don't want to put in "one year" because you could come across as a newbie, you don't want to put in "two years" or "a little less than two years because that's not quite true. The mention of time is important to recruiters since they use "years of experience" both as a yardstick and as a way to collect money from their clients.
Aside from that, recruiters want to go through your letters as quickly as possible. None of them are going to worry about minor variations in writing style let alone spend even two seconds on the nuances of your writing style. Of course, if your writing style is unclear features spelling and grammatical mistakes, that will be a red flag to them. They certainly are not fond of introducing candidates who can't write and spell to prospective employers.