First of all, the visa is invalid now as immigrant visas are single-use. Upon entering the US with the immigrant visa, she immediately became a US permanent resident. Since she got permanent residency by being petitioned by a spouse, if she had been married to you for less than 2 years on the day she entered, she became a conditional permanent resident. Her conditional permanent resident status lasts for 2 years from the date she entered, and this is also reflected on her physical green card. She will need to apply for Removal of Conditions in the 90-day window before then.
The immigrant visa has a line of text on it that says "upon endorsement serves as temporary I-551 evidencing permanent residence for 1 year". The "endorsement" is being stamped (with an entry stamp) when she entered with the immigrant visa. The fact that she entered and got an entry stamp on that immigrant visa automatically turned it into an I-551 (equivalent to a green card for all purposes) valid until exactly 1 year after the date of entry. That "expiration" date of that I-551 does not need to be written anywhere; it is implied by the date of entry (on the entry stamp) and the text on the visa, and nothing they write would change that.
I am not sure what the handwritten date you are referring to means. You did not mention the date she entered (you said the visa was from 2016, but since the immigrant visa is valid for 6 months, she could have entered in 2016 or the first half of 2017), and so it is unclear whether this handwritten date is 1 year or 2 years after she entered. If it is exactly 2 years after she entered, it could be referring to the end of her 2-year period, though the fact that her conditional permanent residency status lasts for 2 years is determined by law and not dependent on what they write. If it is exactly 1 year after she entered, it could refer to the "expiration" of the I-551 that the immigrant visa turned into.