I need to convert this to the standard date time format for further date calculations
You cannot without a day value. What does the data look like in the field that needs to be converted, and what would the desired outcome be? Provide some examples. If you enter 2018-06 into a date field, Access will assume the day value is to be 01, so if you don't have that in the source field, it must be text and not a date. Hence the request for examples of what you have versus what you want.
EDIT - your 2 digit year field will mean that Access will have to decide if 18 is 2018 or the day number, which I presume depends on your regional settings. So you definitely are starting out with bad data.
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