step 1

create integer field for date 

here in our form "dod_mom_nep" and it accepts yyyymmdd like 20710101

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step 2
create hidden valu for englishdate

here in our form "mom_dod_eng"

date(date(/data/engmthmomdod) + /data/dod_mom_nep - (/data/nepmthmondod *100) - 1)
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step 3
create another hidden value

here in our form "nepmthmondod"

int(/data/dod_mom_nep div 100)

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step 4
create another hidden value

here in our form "engmthmomdod"

if(/data/nepmthmondod != "", instance('Months207n')/root/item[nepalimonth=/data/nepmthmondod]/englishdate01, "")