If I understand your question, you have basically solved it already.
Assume you are putting up a Map and asking a user for a starting date and time.
Read this information into Alpha Variables; COMPRESS (or MOVE, or have them REDEFINED as part of a single larger variable) the two separate variables into a single variable, say #alpha-start-timestamp; then use MOVE EDITED to create #start-timestamp.
01 #START-TIMESTAMP (T)
END-DEFINE
*
INPUT #START-TIMESTAMP (EM=YYYY-MM-DD^HH:II:SS) /* use this for example
READ FILE BY TIMESTAMP STARTING FROM #START-TIMESTAMP